I owned an 83 O'day 25 for about 5 years and did a lot of electrical work. One thing I quickly learned was the wiring diagrams I found were really just a suggestion for the installers.
Forget about trying to pull wires that go up to the headliner and forward. O'day apparently ran the wires before gluing in the head liner. Some other runs were slightly easier to replace but not by much.
I basically ran most wires from scratch. I know on one run, on the port side up to the head and v-berth, I had to go from the panel, back along the port side into the quarter berth, up to the access for the winch bolts across to the port side, then between the headliner edge forward.
The area back by the winch access was the only part of the headliner where there was enough space to allow me to run the wires over the quarter berth above the headliner.
Apart from that, from the panel back is relatively easy access for running wires back for the stern light, outboard and to the battery.
I found the O25 to be a great boat and know you'll enjoy yours.