I have two calendars in Outlook: my main one and one I made years ago for a side project. I would like everything in one calendar so I stop missing things. Is there a way to merge them and not copy 300 events by hand?

There are two different things here depending on what you want.

Option 1, just view them together (nothing changes): open the Calendar view, tick both calendars in the left pane, then click the little arrow on the second calendar's tab and choose "View in Overlay Mode". You see both in one grid but they stay separate.

Option 2, a real merge:

1. Click the calendar you want to empty.
2. On the View tab choose Change View > List.
3. Press Ctrl + A to select all items.
4. On the Home tab click Move > Copy to Folder (or hold Ctrl and drag them) and select the destination calendar.

Use Copy first, check the result, then delete the old calendar if you are happy. If you use Move there is no easy undo for 300 items.

Copy to Folder worked. One issue, a few recurring meetings now show twice. Is that normal?

Yes, that happens when the same invite existed in both calendars. In the List view sort by Subject, and delete the extra copies of those few meetings. For a bigger overlap, sort by Start time and look for items with the same subject and time.

Cleaned up 11 duplicates. All good now. Thank you!