Is summer domestic flight more expensive?

I agree with Pilot. Southwest should have the best fares between San Diego and Oakland. You definitely should buy a round-trip because one-way tickets can be expensive. Summertime is typically more expensive for travel because children and students are out of school so people start traveling for vacations. I would suggest that you buy your tickets at least 21 days before travel. Right now just look at fares to see how much they are and if you do happen to find a reasonable fare, go ahead and purchase it.