The O'Day 240 was the replacement for the O'Day 25, only it had the wing keel. I remember going into the new 1986 O'Day 240 at the Boston Boat Shows and I had to bend down a little in the cabin.
Rudy at D&R Marine had bought all the O'Day molds from the fall River O'Day factory after they went out of business. He had a contract with a boat builder in the Bristol RI area to build his boats. Rudy had about six Sunrise 19 sloops built using O'Day 192 molds and also the Sunrise 24 with the O'Day 240 mold. I also went into the Sunrise 24, many years ago when he had his boats on display in Bristol RI.
I think that it would be safe to say that you would have the same amount of headroom on the O'Day 240 as you would on an O'Day 25.
My 1986 O'Day 222 has a large sliding companionway hatch which allows me standing head room in a minimum space which is good enough for me.