Pretty sure all O'days have solid fiberglass. On my 25' O'day the transducer is shooting through the hull centerline just behind the keel. Works well. Test it first by putting the ducer in a plastic bag of water, wet the area your thinking about and placing the ducer in bag of water on the spot and test it.
That one that came with my boat is epoxied to the hull but I just did a quick install of a ducer using toilet bowel seal wax to hold it. I put that one along side of the keel but plan to move it forward of the keel later. It came with my chart plotter/fishfinder and will be a back up to the primary depth sounder. The wax is holding is holding it well.
Lots of threads about sticking a transducer to a hull.