The publisher view in the Activity component is controlled by the "New Task" action layout, not the Task or Event page layout.
If this is an object-specific action, the layout can be edited in Object Manager for the Opportunity, under Buttons, Links and Actions. If this is a global action, the action layout can be edited in Global Actions in Setup. In either case, you'd click into the action itself and then click the Edit Layout button.
You can tell which it type of action this is by viewing the page layout for the Opportunity, navigating to the Lightning and Mobile Actions section, and hovering over the icon for the action in the page layout field navigator. If the hover name starts with "Opportunity", it is an object-specific action.