I changed the oil in my Bravo drives yesterday and noticed that there is no "o" ring on the plug. Should I but some kind
of sealer on them?

Mine have soft metal type washer on them, found similar at my autoparts. if they are grooved for o-ring then best to use o-rings.

I wouldn't trust thread sealer in this application

Check in the hole where the plug came out of. Alot of times the o-ring will stick in there when you pull the plug out.

Yep, its probably still in the hole.

YOU MUST remove ALL of the old one, and replace it with a new one. They come in a little box or clear baggy, and I think you can get them at Boater's World now. If not your Merc Dealer will BOHICA you for a couple. They are either orange (old style) or blue (new style). They will leak if not seated properly. You need one on the top vent and one on the bottom drain, mandatory. I would not put any sealer or anything else on there. They are hard enough to remove with such a shallow crevice for the screwdriver.

Sidwayz is absolutely right, I've found that if they are old they will get really soft and just flow into the corners of the opening thus not allowing the new one to seal either. You need to insure that the contact area is clean. You may need a pick of some type to get into the corners. It will definitely leak without one.

Just lived the experience 3 weeks ago. You can get them at BAM Marine on-line. I had the plug & washer delivered the next day.

Mike