Lakes of Oklahoma book - The OWRB put together a beautiful book that shows most of the lakes in Oklahoma. Many show the various facilities available at each one (camping, boat ramps, etc) and some even show depth lines. You can go by and pick up a free copy, or you can view the maps online. They also now have an interactive lake viewer.
National Water Information System Mapper - The USGS has upgraded their site some. This is a zoomable map with pins for every USGS measurement station in the country. Just zoom to the area you're interested in, select the monitoring station, and you can get to the current and historical records.
National Map Streamer - This is really sweet, zoom in on the map to find a river you're interested in, click on it somewhere, and it shows you where the water comes from / goes to (click upstream or downstream at the top).
OKC Kayak website - They have a calendar of upcoming events. Check it for Full Moon Paddles, Demo Days, and other great activities.
TravelOK Lake Current Conditions - There is a drop-down with most Oklahoma lakes listed. Select one, click Go, and it gives you the most recent info they have on that lake. Some don't have much info, but others have fishing reports and lake level / water conditions. Many have a phone number for the lake manager. They also give blue-green algae reports for each lake.