How do you say see at oktoberfest in German?

You would not say "see" at Oktoberfest in German. You would use "Prosit!" which means "Cheers!"

If you want to specifically say "see you", you would use "Bis bald!" (see you soon) or "Bis dann!" (see you then).

However, "Prosit!" is the most common and appropriate phrase to use when toasting at Oktoberfest.