diff --git a/bson/__init__.py b/bson/__init__.py
index 972b184015..f61a7ddf75 100644
--- a/bson/__init__.py
+++ b/bson/__init__.py
@@ -1406,7 +1406,7 @@ def has_c() -> bool:
     return _USE_C
 
 
-def _after_fork():
+def _after_fork() -> None:
     """Releases the ObjectID lock child."""
     if ObjectId._inc_lock.locked():
         ObjectId._inc_lock.release()
diff --git a/bson/binary.py b/bson/binary.py
index 0727bd17c0..f8a475f8a3 100644
--- a/bson/binary.py
+++ b/bson/binary.py
@@ -365,5 +365,5 @@ def __hash__(self) -> int:
     def __ne__(self, other: Any) -> bool:
         return not self == other
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"Binary({bytes.__repr__(self)}, {self.__subtype})"
diff --git a/bson/code.py b/bson/code.py
index 37b2aa85d4..26bed0103d 100644
--- a/bson/code.py
+++ b/bson/code.py
@@ -86,7 +86,7 @@ def scope(self) -> Optional[Mapping[str, Any]]:
         """Scope dictionary for this instance or ``None``."""
         return self.__scope
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"Code({str.__repr__(self)}, {self.__scope!r})"
 
     def __eq__(self, other: Any) -> bool:
diff --git a/bson/codec_options.py b/bson/codec_options.py
index f146898dfd..9c511b5d6f 100644
--- a/bson/codec_options.py
+++ b/bson/codec_options.py
@@ -172,7 +172,7 @@ def _validate_type_encoder(self, codec: _Codec) -> None:
                 )
                 raise TypeError(err_msg)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return "{}(type_codecs={!r}, fallback_encoder={!r})".format(
             self.__class__.__name__,
             self.__type_codecs,
@@ -465,7 +465,7 @@ def _options_dict(self) -> Dict[str, Any]:
                 "datetime_conversion": self.datetime_conversion,
             }
 
-        def __repr__(self):
+        def __repr__(self) -> str:
             return f"{self.__class__.__name__}({self._arguments_repr()})"
 
         def with_options(self, **kwargs: Any) -> "CodecOptions":
diff --git a/bson/datetime_ms.py b/bson/datetime_ms.py
index c422d6e379..6b9472b5b6 100644
--- a/bson/datetime_ms.py
+++ b/bson/datetime_ms.py
@@ -111,12 +111,12 @@ def __int__(self) -> int:
 # Timezones are hashed by their offset, which is a timedelta
 # and therefore there are more than 24 possible timezones.
 @functools.lru_cache(maxsize=None)
-def _min_datetime_ms(tz=datetime.timezone.utc):
+def _min_datetime_ms(tz: datetime.timezone = datetime.timezone.utc) -> int:
     return _datetime_to_millis(datetime.datetime.min.replace(tzinfo=tz))
 
 
 @functools.lru_cache(maxsize=None)
-def _max_datetime_ms(tz=datetime.timezone.utc):
+def _max_datetime_ms(tz: datetime.timezone = datetime.timezone.utc) -> int:
     return _datetime_to_millis(datetime.datetime.max.replace(tzinfo=tz))
 
 
diff --git a/bson/dbref.py b/bson/dbref.py
index 1141e7ba12..1bd4cadcc0 100644
--- a/bson/dbref.py
+++ b/bson/dbref.py
@@ -101,7 +101,7 @@ def as_doc(self) -> SON[str, Any]:
         doc.update(self.__kwargs)
         return doc
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         extra = "".join([f", {k}={v!r}" for k, v in self.__kwargs.items()])
         if self.database is None:
             return f"DBRef({self.collection!r}, {self.id!r}{extra})"
diff --git a/bson/decimal128.py b/bson/decimal128.py
index 0e24b5bbae..fd39e94705 100644
--- a/bson/decimal128.py
+++ b/bson/decimal128.py
@@ -296,7 +296,7 @@ def __str__(self) -> str:
             return "NaN"
         return str(dec)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"Decimal128('{str(self)}')"
 
     def __setstate__(self, value: Tuple[int, int]) -> None:
diff --git a/bson/json_util.py b/bson/json_util.py
index bc566fa982..82604f382f 100644
--- a/bson/json_util.py
+++ b/bson/json_util.py
@@ -224,7 +224,7 @@ class JSONOptions(CodecOptions):
     datetime_representation: int
     strict_uuid: bool
 
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args: Any, **kwargs: Any):
         """Encapsulates JSON options for :func:`dumps` and :func:`loads`.
 
         :Parameters:
diff --git a/bson/max_key.py b/bson/max_key.py
index eb5705d378..83278087b0 100644
--- a/bson/max_key.py
+++ b/bson/max_key.py
@@ -50,5 +50,5 @@ def __ge__(self, dummy: Any) -> bool:
     def __gt__(self, other: Any) -> bool:
         return not isinstance(other, MaxKey)
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return "MaxKey()"
diff --git a/bson/min_key.py b/bson/min_key.py
index 2c8f73d560..50011df6e5 100644
--- a/bson/min_key.py
+++ b/bson/min_key.py
@@ -50,5 +50,5 @@ def __ge__(self, other: Any) -> bool:
     def __gt__(self, dummy: Any) -> bool:
         return False
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return "MinKey()"
diff --git a/bson/objectid.py b/bson/objectid.py
index 966fd9f94b..d3afe3cd3c 100644
--- a/bson/objectid.py
+++ b/bson/objectid.py
@@ -243,7 +243,7 @@ def __setstate__(self, value: Any) -> None:
     def __str__(self) -> str:
         return binascii.hexlify(self.__id).decode()
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"ObjectId('{str(self)}')"
 
     def __eq__(self, other: Any) -> bool:
diff --git a/bson/raw_bson.py b/bson/raw_bson.py
index f48016909c..d5dbe8fbf9 100644
--- a/bson/raw_bson.py
+++ b/bson/raw_bson.py
@@ -173,7 +173,7 @@ def __eq__(self, other: Any) -> bool:
             return self.__raw == other.raw
         return NotImplemented
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return "{}({!r}, codec_options={!r})".format(
             self.__class__.__name__,
             self.raw,
diff --git a/bson/regex.py b/bson/regex.py
index c06e493f38..fe852fdfce 100644
--- a/bson/regex.py
+++ b/bson/regex.py
@@ -115,7 +115,7 @@ def __eq__(self, other: Any) -> bool:
     def __ne__(self, other: Any) -> bool:
         return not self == other
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"Regex({self.pattern!r}, {self.flags!r})"
 
     def try_compile(self) -> "Pattern[_T]":
diff --git a/bson/son.py b/bson/son.py
index 482e8d2584..7be749ceca 100644
--- a/bson/son.py
+++ b/bson/son.py
@@ -71,7 +71,7 @@ def __new__(cls: Type["SON[_Key, _Value]"], *args: Any, **kwargs: Any) -> "SON[_
         instance.__keys = []
         return instance
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         result = []
         for key in self.__keys:
             result.append(f"({key!r}, {self[key]!r})")
diff --git a/bson/timestamp.py b/bson/timestamp.py
index 5591b60e41..168f2824df 100644
--- a/bson/timestamp.py
+++ b/bson/timestamp.py
@@ -111,7 +111,7 @@ def __ge__(self, other: Any) -> bool:
             return (self.time, self.inc) >= (other.time, other.inc)
         return NotImplemented
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return f"Timestamp({self.__time}, {self.__inc})"
 
     def as_datetime(self) -> datetime.datetime:
diff --git a/pymongo/_csot.py b/pymongo/_csot.py
index 7a5a8a7302..9ad477a249 100644
--- a/pymongo/_csot.py
+++ b/pymongo/_csot.py
@@ -14,6 +14,8 @@
 
 """Internal helpers for CSOT."""
 
+from __future__ import annotations
+
 import functools
 import time
 from collections import deque
@@ -72,7 +74,7 @@ def __init__(self, timeout: Optional[float]):
         self._timeout = timeout
         self._tokens: Optional[Tuple[Token, Token, Token]] = None
 
-    def __enter__(self):
+    def __enter__(self) -> _TimeoutContext:
         timeout_token = TIMEOUT.set(self._timeout)
         prev_deadline = DEADLINE.get()
         next_deadline = time.monotonic() + self._timeout if self._timeout else float("inf")
@@ -81,7 +83,7 @@ def __enter__(self):
         self._tokens = (timeout_token, deadline_token, rtt_token)
         return self
 
-    def __exit__(self, exc_type, exc_val, exc_tb):
+    def __exit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> None:
         if self._tokens:
             timeout_token, deadline_token, rtt_token = self._tokens
             TIMEOUT.reset(timeout_token)
@@ -97,7 +99,7 @@ def apply(func: F) -> F:
     """Apply the client's timeoutMS to this operation."""
 
     @functools.wraps(func)
-    def csot_wrapper(self, *args, **kwargs):
+    def csot_wrapper(self: Any, *args: Any, **kwargs: Any) -> F:
         if get_timeout() is None:
             timeout = self._timeout
             if timeout is not None:
diff --git a/pymongo/auth_aws.py b/pymongo/auth_aws.py
index 2f7dcb857f..a327016d73 100644
--- a/pymongo/auth_aws.py
+++ b/pymongo/auth_aws.py
@@ -23,7 +23,7 @@
 except ImportError:
 
     class AwsSaslContext:  # type: ignore
-        def __init__(self, credentials):
+        def __init__(self, credentials: MongoCredential):
             pass
 
     _HAVE_MONGODB_AWS = False
@@ -35,11 +35,11 @@ def __init__(self, credentials):
     set_use_cached_credentials(True)
 except ImportError:
 
-    def set_cached_credentials(creds):
+    def set_cached_credentials(creds: Optional[AwsCredential]) -> None:
         pass
 
 
-from typing import TYPE_CHECKING, Any, Mapping
+from typing import TYPE_CHECKING, Any, Mapping, Optional, Type
 
 import bson
 from bson.binary import Binary
@@ -54,7 +54,7 @@ def set_cached_credentials(creds):
 
 class _AwsSaslContext(AwsSaslContext):  # type: ignore
     # Dependency injection:
-    def binary_type(self):
+    def binary_type(self) -> Type[Binary]:
         """Return the bson.binary.Binary type."""
         return Binary
 
diff --git a/pymongo/change_stream.py b/pymongo/change_stream.py
index a40b5b2f14..ed3031ef52 100644
--- a/pymongo/change_stream.py
+++ b/pymongo/change_stream.py
@@ -257,7 +257,7 @@ def _run_aggregation_cmd(
             cmd.get_cursor, self._target._read_preference_for(session), session
         )
 
-    def _create_cursor(self):
+    def _create_cursor(self) -> CommandCursor:
         with self._client._tmp_session(self._session, close=False) as s:
             return self._run_aggregation_cmd(session=s, explicit_session=self._session is not None)
 
diff --git a/pymongo/collation.py b/pymongo/collation.py
index bdc996be1b..bada2d9417 100644
--- a/pymongo/collation.py
+++ b/pymongo/collation.py
@@ -199,7 +199,7 @@ def document(self) -> Dict[str, Any]:
         """
         return self.__document.copy()
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         document = self.document
         return "Collation({})".format(", ".join(f"{key}={document[key]!r}" for key in document))
 
diff --git a/pymongo/collection.py b/pymongo/collection.py
index fbbe7fb593..6b2b16db77 100644
--- a/pymongo/collection.py
+++ b/pymongo/collection.py
@@ -2481,7 +2481,7 @@ def create_search_indexes(
         if comment is not None:
             kwargs["comment"] = comment
 
-        def gen_indexes():
+        def gen_indexes() -> Iterator[Mapping[str, Any]]:
             for index in models:
                 if not isinstance(index, SearchIndexModel):
                     raise TypeError(
diff --git a/pymongo/event_loggers.py b/pymongo/event_loggers.py
index 248dfb17bd..70c386ab04 100644
--- a/pymongo/event_loggers.py
+++ b/pymongo/event_loggers.py
@@ -179,7 +179,7 @@ class ConnectionPoolLogger(monitoring.ConnectionPoolListener):
     def pool_created(self, event: monitoring.PoolCreatedEvent) -> None:
         logging.info(f"[pool {event.address}] pool created")
 
-    def pool_ready(self, event):
+    def pool_ready(self, event: monitoring.PoolReadyEvent) -> None:
         logging.info(f"[pool {event.address}] pool ready")
 
     def pool_cleared(self, event: monitoring.PoolClearedEvent) -> None:
diff --git a/pymongo/ocsp_cache.py b/pymongo/ocsp_cache.py
index b0ac4d654f..033a7b607a 100644
--- a/pymongo/ocsp_cache.py
+++ b/pymongo/ocsp_cache.py
@@ -19,7 +19,7 @@
 from collections import namedtuple
 from datetime import datetime as _datetime
 from datetime import timezone
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any, Dict
 
 from pymongo.lock import _create_lock
 
@@ -35,8 +35,8 @@ class _OCSPCache:
         ["hash_algorithm", "issuer_name_hash", "issuer_key_hash", "serial_number"],
     )
 
-    def __init__(self):
-        self._data = {}
+    def __init__(self) -> None:
+        self._data: Dict[Any, OCSPResponse] = {}
         # Hold this lock when accessing _data.
         self._lock = _create_lock()
 
@@ -77,7 +77,10 @@ def __setitem__(self, key: OCSPRequest, value: OCSPResponse) -> None:
             # Cache new response OR update cached response if new response
             # has longer validity.
             cached_value = self._data.get(cache_key, None)
-            if cached_value is None or cached_value.next_update < value.next_update:
+            if cached_value is None or (
+                cached_value.next_update is not None
+                and cached_value.next_update < value.next_update
+            ):
                 self._data[cache_key] = value
 
     def __getitem__(self, item: OCSPRequest) -> OCSPResponse:
@@ -92,6 +95,8 @@ def __getitem__(self, item: OCSPRequest) -> OCSPResponse:
             value = self._data[cache_key]
 
             # Return cached response if it is still valid.
+            assert value.this_update is not None
+            assert value.next_update is not None
             if (
                 value.this_update
                 <= _datetime.now(tz=timezone.utc).replace(tzinfo=None)
diff --git a/pymongo/topology.py b/pymongo/topology.py
index 6fd1138fb2..5b4197bc16 100644
--- a/pymongo/topology.py
+++ b/pymongo/topology.py
@@ -890,7 +890,7 @@ def __repr__(self) -> str:
             msg = "CLOSED "
         return f"<{self.__class__.__name__} {msg}{self._description!r}>"
 
-    def eq_props(self):
+    def eq_props(self) -> Tuple[Tuple[_Address, ...], Optional[str], Optional[str], str]:
         """The properties to use for MongoClient/Topology equality checks."""
         ts = self._settings
         return (tuple(sorted(ts.seeds)), ts.replica_set_name, ts.fqdn, ts.srv_service_name)
diff --git a/tools/ocsptest.py b/tools/ocsptest.py
index bba84252be..702f15ee99 100644
--- a/tools/ocsptest.py
+++ b/tools/ocsptest.py
@@ -25,7 +25,7 @@
 logging.basicConfig(format=FORMAT, level=logging.DEBUG)
 
 
-def check_ocsp(host, port, capath):
+def check_ocsp(host: str, port: int, capath: str) -> None:
     ctx = get_ssl_context(
         None,  # certfile
         None,  # passphrase
@@ -47,7 +47,7 @@ def check_ocsp(host, port, capath):
         s.close()
 
 
-def main():
+def main() -> None:
     parser = argparse.ArgumentParser(description="Debug OCSP")
     parser.add_argument("--host", type=str, required=True, help="Host to connect to")
     parser.add_argument("-p", "--port", type=int, default=443, help="Port to connect to")
diff --git a/tox.ini b/tox.ini
index c268fd1f4c..11913a8832 100644
--- a/tox.ini
+++ b/tox.ini
@@ -86,7 +86,7 @@ deps =
     certifi; platform_system == "win32" or platform_system == "Darwin"
     typing_extensions
 commands =
-    mypy --install-types --non-interactive bson gridfs tools pymongo
+    mypy --install-types --non-interactive --disallow-untyped-defs bson gridfs tools pymongo
     mypy --install-types --non-interactive --disable-error-code var-annotated --disable-error-code attr-defined --disable-error-code union-attr --disable-error-code assignment --disable-error-code no-redef --disable-error-code index --allow-redefinition --allow-untyped-globals --exclude "test/mypy_fails/*.*" --exclude "test/conftest.py" test
     mypy --install-types --non-interactive test/test_typing.py test/test_typing_strict.py