In my opinion, Oak is the best 1 v 1 demigod in the game by far. But he doesn't excel as much in group battles: your goal should be to never engage Oak 1 v 1. Attack Oak with an ally, even if he has an ally himself. If you both target the same enemy, you should be able to win, even if he has an ally because your heals should keep your team alive longer than them.
Also, use the tactics that suit your demigod. Oak's damage, armor, and idols make him well-suited for a straight-up fight. Sedna's speed and survivability make her well-suited for hunting down demigods when they're weak and chasing them even into their base and further well-suited for quickly capturing flags, and further well-suited for ambushing demigods who are under assault by one of your allies.