It depends on what you are looking for. If you want season tickets, then you can buy them directly from the OU ticket office.
I can guarantee you, that if you show up on game day without tickets, you'll get the cheapest price on tickets, by buying the outside the stadium within an hour of kickoff. But we all have our priorities. It also depends on how many tickets you are buying. And how much of the pregame you want to see. I like being there when the kickers walk out onto the field for pregame. You pay a little more for tickets the earlier you buy. But there are ample tickets on game day for any OU game in Norman. I've never paid over face for a game in Norman. But I usually am buying only one or two. Larger numbers are more challenging.
You can also check message boards like this one for tickets. Here and at least one other site have specific boards with people selling their tickets because they can't go for some reason.
Another good source is the OU official site. They charge a small fee to sell the seats of season ticket holders who aren't going to the game. It's the most reliable source.
My personal experience is that ticket sites like Stubhub are the most expensive seats, because the sites charge a huge fee per seat. And if you've never used that site before, you will be dogged by spam email or texts or both at least weekly in the future.
One other thing. I would avoid sitting in the upper deck on the east side. West side upper deck is terrific. But the east side is way up high. And if it's a sunny day, you'll have the sun in your face all day long for any day games.