[BEAR.]

I don't think anything would've really changed. Oakheart was Crookedstar's first choice, and when he rejected the proposal, it just made Timberfur deputy until he resigned (or died, since neither have been confirmed.)

Other than getting the rank a bit sooner and reducing the amount of deputies RiverClan had, I highly doubt anything noteworthy would have changed within the series. The Erins would have still had him get squished by a rock and Crookedstar would have still died of Greencough at the end of A Dangerous Path.

All in all, there was never anything particularly significant about Oakheart being deputy of RiverClan. He didn't do a lot besides participate in battles and (as a warrior) took on a mate in another Clan. Their relationship by then was already shattered and their loyalties were to their own Clan. Tensions would've remained the same.

Realistically, you could just take Oakheart out of the books entirely and overall, not much would change other than the removal of a few half-Clan cats and changes within the high ranks. It's likely Leopardstar wouldn't have felt any remorse at all for killing the half-Clan cats because there wouldn't be anyone to protect Stormfur and Feathertail.