I have the Yamaha T9.9 XL High Thrust OB on my O'day 25. Great engine.
http://
I have the same motor on my Venture 21, barely need to crank it above idle. It was on my Ranger 26, which is a bit heavier than you O'Day, I think, b/c it ran at full throttle, or nearly so most of the time, if I was motoring, but it had plenty of thrust for maneuvering around a marina. It got something like 1 gal per hour.
My first time out, crossing the mouth of Grand Traverse Bay, and motoring b/c the wind was dead on, and my rigging fouled, I came too close to running out of fuel with only one of my 2-6gal fuel tanks full. I learned that lesson fast.