Reserviors like Oahe and Sakakawea have tons of places for the fish to live. The key I have always felt was to first determine the type of structure they are using. Is it shallow, is it mid depth, is it real deep. Are they on flats or relating to the numerous points in a reservior…
So first thing is to spend a lot of time graphing (and potentially using a camera). If the fish are 15 foot or deeper, you should have no problem spotting them with a good fish finder. I would suggest a color unit as many times the fish are holding right tight to the bottom and on a color unit that is much easier to interpret. I use the Lowrance LCX 112C for most of my searching. The one thing about the 112C (and several other of the high end units) is that they have fast scroll and ping speeds, so you can move along the structure pretty fast (I would say up to 10 mph) and still see fish.
If the fish are up off the bottom a little, that is great. These are normally active feeding fish – ones you should go after right away with a fairly agressive technique (cranks or spinners). If they are tight to the bottom, they are probably a little less active, I might try Slow Death or just a bare leech on a bottom bouncer.
If the fish are shallow, they will be a little harder to find because you will have to fish to find them. Concentrate on high percentage areas (rocky shoreline areas, sections of the reservior where the old channel comes close to shore, points that extend out a long way into the reservior).
Once you have located the fish, the next step is to find the winning fish. I have found over the years that different areas of a reservior will hold bigger fish. So once your find out the general depth the fish are holding in (from above), now cover water, look at a bunch of sections of the reservior and try to determine where the biggest fish are hanging out.
Lastly, once you find the fish, remember that reservior fish are known to move, so find a bunch of spots and set up a milk run – that way you have options on tournament day.