For Oahe here is a guide that was recommended to me. Wife and I have been with him twice now and definitely recommend him.
https://m.facebook.com/shanecowanguideservice/
Wife and I will be finishing up our trip to Oahe when you are there.
If you can’t find a guide; there is a boat ramp right by the highway 212 bridge that if you fish around it/that area you should be able to catch a smallmouth from the bank.
If you see a blue Charger bassboat flag us down and we could get you out with us as well. We are FAR from knowing Oahe as we are still learning the lake but should be able to get you a smallie or two.