diff --git a/pw_protobuf/py/pw_protobuf/options.py b/pw_protobuf/py/pw_protobuf/options.py
index d9ffca6..9fc301b 100644
--- a/pw_protobuf/py/pw_protobuf/options.py
+++ b/pw_protobuf/py/pw_protobuf/options.py
@@ -26,9 +26,11 @@
 _SINGLE_LINE_COMMENT_RE = re.compile(r'//.*?$', flags=re.MULTILINE)
 _SHELL_STYLE_COMMENT_RE = re.compile(r'#.*?$', flags=re.MULTILINE)
 
+# A list of (proto field path, Options) tuples.
+FieldOptions = List[Tuple[str, Options]]
 
-def load_options_from(options: List[Tuple[str, Options]],
-                      options_file_name: Path):
+
+def load_options_from(options: FieldOptions, options_file_name: Path):
     """Loads a single .options file for the given .proto"""
     with open(options_file_name) as options_file:
         # Read the options file and strip all styles of comments before parsing.
@@ -52,9 +54,9 @@
 
 
 def load_options(include_paths: List[Path],
-                 proto_file_name: Path) -> List[Tuple[str, Options]]:
+                 proto_file_name: Path) -> FieldOptions:
     """Loads the .options for the given .proto."""
-    options: List[Tuple[str, Options]] = []
+    options: FieldOptions = []
 
     for include_path in include_paths:
         options_file_name = include_path / proto_file_name.with_suffix(
@@ -65,7 +67,7 @@
     return options
 
 
-def match_options(name: str, options: List[Tuple[str, Options]]) -> Options:
+def match_options(name: str, options: FieldOptions) -> Options:
     """Return the matching options for a name."""
     matched = Options()
     for name_glob, mask_options in options:
diff --git a/pw_protobuf/py/pw_protobuf/proto_tree.py b/pw_protobuf/py/pw_protobuf/proto_tree.py
index db0ca42..687b16e 100644
--- a/pw_protobuf/py/pw_protobuf/proto_tree.py
+++ b/pw_protobuf/py/pw_protobuf/proto_tree.py
@@ -530,8 +530,9 @@
                                response_node))
 
 
-def _populate_fields(proto_file, global_root: ProtoNode,
-                     package_root: ProtoNode, proto_options) -> None:
+def _populate_fields(proto_file: descriptor_pb2.FileDescriptorProto,
+                     global_root: ProtoNode, package_root: ProtoNode,
+                     proto_options: Optional[options.FieldOptions]) -> None:
     """Traverses a proto file, adding all message and enum fields to a tree."""
     def populate_message(node, message):
         """Recursively populates nested messages and enums."""
@@ -558,7 +559,9 @@
         _add_service_methods(global_root, package_root, service_node, service)
 
 
-def _build_hierarchy(proto_file):
+def _build_hierarchy(
+    proto_file: descriptor_pb2.FileDescriptorProto
+) -> Tuple[ProtoPackage, ProtoPackage]:
     """Creates a ProtoNode hierarchy from a proto file descriptor."""
 
     root = ProtoPackage('')
@@ -590,8 +593,10 @@
     return root, package_root
 
 
-def build_node_tree(file_descriptor_proto,
-                    proto_options=None) -> Tuple[ProtoNode, ProtoNode]:
+def build_node_tree(
+    file_descriptor_proto: descriptor_pb2.FileDescriptorProto,
+    proto_options: Optional[options.FieldOptions] = None
+) -> Tuple[ProtoNode, ProtoNode]:
     """Constructs a tree of proto nodes from a file descriptor.
 
     Returns the root node of the entire proto package tree and the node