Long creek!?!? I even know where that is! I can't speak for Grant County, but I have OCed ALL over Hermiston and some in Pendleton without incident. I don't see that Grant County would be much different. Someone I was talking to at the place I shoot was saying that when he was a kid, he OCed to his senior prom in Pilot Rock - nobody thought anything of it.
As to the OC at age 18, just read ORS Chapter 166. http://www.leg.state.or.us/ors/166.html