Is there a convention that the winner gets to name the battle?

Actually it is not that the winner gets to name the battle but more correctly that the battle is known by the victors name for it. In their reports to their headquarters the Union officers would send information about Ocean Pond while the Confederates would have sent their reports to their HQs about Olustee.

The newspapers of the time would use whatever their leanings were, Union papers would refer to the battle at Ocean Pond while the Confederate papers would refer to Olustee, both the same battle. Later when the authors of the history texts compiled their information they would use the name that the winnig side had given to teh battle.

This applies to most other battles as well. (Mannassas - Bull Run, Antietam - Sharpsburg, etc..)