Back when I was playing Burning Oath (pitch version) I won most of my matches against Bug fish through the oath plan. A single misdirection gets you the turn to oath, or a time walk. The few times oath didn't work their hand was light enough that I was able to storm them out with ease, I can easily see the same being said for tezz vaulting.
I know Bug is better positioned to fight oath compared to other fish decks, but it is still not an easy fight, and a surge in oath definitely makes Bug worse in the meta. Not to mention it is also a dog to a resolved show and tell, so unless every fish deck main decks a karakas I don't see the match up being in their favor any time soon.
And if they do decide to run Karakas, Oath decks can just run Progenitus.