OK hypothetical here:
looking at the entry for the Oathbound Paladin:
Paladins who take up an oath may make a sacred promise to their god or temple to perform some specific and grand action associated with the oath. For example, an oathbound paladin who takes the Oath of Vengeance may be tasked with killing the orc warlord who razed her home city, while a paladin with the Oath against the Wyrm may be asked to secure a nonaggression pact with a family of dragons. When a paladin completes the sacred promise, the oath is fulfilled, and she may abandon the oath if she so chooses; she may then select another oath or become a standard paladin or a different paladin archetype.
so based on this what happens if someone else fulfills the oath.
say My Paladin is joining an adventuring group (has not joined yet) but before I join the party kills the creature that I have the Oath to kill.
thus is the oath fulfilled? I just made a vow or promise to my god to defeat this creature yet I get there and it was already killed by someone else.
technically it is oath fulfilled but I would think that there would be some repercussions as I did not fulfill the oath nor was I even present when it was fulfilled.
(note that this is NOT PFS, rather a PbP game where I would be joining later in the adventure )
This is really a GM call. My take FWIW:
You haven't failed, nor have you fulfilled your oath. Your oath has become invalid and impossible to complete. You would need to make a fresh oath. I cannot see that their will be any repercussions as you didn't fail due to either action or inaction on your part.