The pressure on Oxbow varries. I'm assuming you are banking it? I would not bother fishing by the boat ramp, that is where folks plunk, and there might be a few people there. Walk up river from the boat ramp, there is a good trail and you'll find room. Another good spot to swing flies is the naked lady hole. It is the hole below the ramp. As you look down river from the boat ramp there is an island at the far end of the island the naked lady hole begins. You can park closer to it and walk down the trail to it. What is a good distance? Depends....are you single handing or spey fishing? If I show up and there are folks drift fishing or bobber fishing, I just move on to another hole. If they are swinging spoons, then it's a better fit. You sound like a person who doesn't mind walking, or wading so I'm sure you will find room. Fin clipped fish can be kept, but alway check because thing can change. Pick up a copy of the regs and read the posting at the park (yellow odfw signs stapled to trees by the river).
An other option for you might be Barton Park on the Clakamas River. There is a good strech of water to swing flies from over there. Good luck.