diff --git a/plinth/operation.py b/plinth/operation.py
index ba6591e69..8d61ed09d 100644
--- a/plinth/operation.py
+++ b/plinth/operation.py
@@ -127,7 +127,7 @@ class Operation:
         return None
 
     @property
-    def translated_message(self):
+    def translated_message(self) -> str:
         """Return a message about status of operation after translating.
 
         Must be called from a web request (UI) thread with user language set so
@@ -143,7 +143,7 @@ class Operation:
 
         return message
 
-    def _update_notification(self):
+    def _update_notification(self) -> None:
         """Show an updated notification if needed."""
         if not self.show_notification:
             return
@@ -169,7 +169,7 @@ class Operation:
 class OperationsManager:
     """Global handler for all operations and their results."""
 
-    def __init__(self):
+    def __init__(self) -> None:
         """Initialize the object."""
         self._operations: list[Operation] = []
         self._current_operation: Operation | None = None
@@ -182,7 +182,7 @@ class OperationsManager:
         # when done from the same thread which holds the lock.
         self._lock = threading.RLock()
 
-    def new(self, *args, **kwargs):
+    def new(self, *args, **kwargs) -> Operation:
         """Create a new operation instance and add to global list."""
         with self._lock:
             kwargs['on_complete'] = self._on_operation_complete
@@ -192,7 +192,7 @@ class OperationsManager:
             self._schedule_next()
             return operation
 
-    def _on_operation_complete(self, operation):
+    def _on_operation_complete(self, operation: Operation):
         """Trigger next operation. Called from within previous thread."""
         logger.debug('%s: on_complete called', operation)
         with self._lock:
@@ -203,7 +203,7 @@ class OperationsManager:
 
             self._schedule_next()
 
-    def _schedule_next(self):
+    def _schedule_next(self) -> None:
         """Schedule the next available operation."""
         with self._lock:
             if self._current_operation:
@@ -216,7 +216,7 @@ class OperationsManager:
                     operation.run()
                     break
 
-    def filter(self, app_id):
+    def filter(self, app_id: str) -> list[Operation]:
         """Return operations matching a pattern."""
         with self._lock:
             return [
@@ -224,7 +224,7 @@ class OperationsManager:
                 if operation.app_id == app_id
             ]
 
-    def collect_results(self, app_id):
+    def collect_results(self, app_id: str) -> list[Operation]:
         """Return the finished operations for an app."""
         results: list[Operation] = []
         remaining: list[Operation] = []