I looked closely at an O'Day 25 before buying the Catalina 25 that I had. It was actually very similar to my Catalina 25 in specs: fin keel and tall rig
The O'Day had a bit larger interior (mostly a couple of inches more headroom) than the Catalina, but overall construction methods and quality felt pretty similar. In some ways I actually prefer the simplicity of the lower budget cruising boats that leave out headliners (those hide problems) and have simple fiberglass interior liners that are easy to maintain, but hard to customize.
We passed on the O'Day primarily due to a crack in the mast that was not very well dealt with. I don't think that is an O'Day issue in general, it was probably just something on this boat.
I've raced against a few O'Day 25s in the Catalina and performance seems to be pretty similar. I think that size of boat can be a lot of fun to sail and race.