Reduced sail on an O'Day 25
I too own an O'Day 25. You didn't mention what year it was, so I'm going to assume it's an older one (mine is a 1976 model). When I first got my boat, I experimented around with different configurations for reduced sail. I didn't know much, so I didn't know many different things to try, and eventually settled into three configurations:1 Full sail - mainsail and 150 genoa2 Reduced for light wind - 150 genoa only3 Reduce for heavy wind - mainsail onlyExperience since then (I've had the boat 2 1/2 years, and sail a LOT) has shown that of the above setups, only the first is right, and that only part of the time.What follows is what I BELIEVE from experience. I could be mistaken about any of it, and if someone with more experience than me knows differently, or even BELIEVES differently, please feel free to correct me.My boat is designed with the jib sail as the main "engine". The main serves basically to stablize the boat. If you take the main down, the boat (as mentioned by others) tends to lee helm badly. It is also much less responsive to any attempts to bring it into the wind. As a side note, when I sail, I set the jib for what I believe to be optimum, and then set the main to best serve the jib (IE set to allow the jib full wind instead of "stealing" the jib's wind). This seems to work very well for us.When we take the jib down, and sail with only the main, the boat reacts to the rudder very poorly on all directions, but especially when the wind is abeam or on a run. The boat sideslips terribly, and is very unresponsive when turning.Some of the stitching on the Genoa came loose, so we were using the largely unused smaller jib (110) while it was being repaired. I was amazed at the difference. Gone were the 25-35 thrillin heels in the wind, but the boat pointed better, was more responsive and less tiring.I'm one of these kind of people that like to have everything working the way it was designed to work, and then start building my understanding of things after that. When I got my boat, I noticed the grommet holes in the main for reefing, but really didn't use them. The on time I tried to reef (early one), I just tying small ropes through them. Seemed a lot of trouble, and I didn't see much difference (at the time, I was only using the main, now "not making much difference" makes sense). I also noticed several blocks on the boom that weren't being used. I finally found that the O'Day 25 came equipped with a jiffy-reefing system. I did some some research, and found that all that was really missing on mine was a snatch block and some line. I got the jiffy reef system reinstalled, and learned to use it.One day in a sudden storm on our lake (that fortunately only lasted about 10 minutes), we had a really hard time getting the Genoa down. After that, I installed a downhaul. It's made a world of difference for my wife who usually drops sails, and me who is usually the one getting chewed out for not holding into the wind long enough, or losing my patience when she's taking too long. Now, she really likes me "helping" her pull the jib down.Now, to tie all that together, I learned a bunch of lessons that make my sailing experience better. I have two basic setups that I go with:15 knots or less150 genoa & main sail. I reef the main sail if I find myself having to regularly over compensate for weather helm. If that doesn't help, or if the winds pick up significantly, we drop the Genoa and put up the 110. The downhaul helps tremendously with dropping the jib.15 knots or more110 jib, and main sail reefed. We unreef if the winds let up, or we feel comfortable doing so.The only time I sail with only one sail (the main) is when we decide to sail into our marina slip instead of motoring. I do this because the prevailing winds usually are at our back and slightly from the port side, and it's blocked well enough to prevent any surprises. The one time I made the mistake of leaving it up all the way into the slip, when I turned into the slip, the boat kept sliding sideways. I didn't bang it up, but only because my brother was on board, and jumped on to the dock to push me over. Now, I power until right before I turn, have the wife quickly drop the sail, and coast in. I NEVER do this single handed, for that I use the motor.All this to say, I don't believe the O'Day 25 was designed to sail well with only one sail. I think you're better off putting up the proper jib, then reefing the mainsail as needed. If someone had told me that 2 years ago, I would have smiled and kept doing what I did back then. However, back then I was just a new boat owner, now I'm more of a sailor.