I'm new to this site so I hope I am not repeating a question.
I am replacing the subfloor in my bathroom. I have cut it out and there is about 2 inches from the wall where the circular saw got as close as it could. The floor is 3/4 OSB and the joist are 12 inches centre.
My question is, do I need to put wood between the joist where the new OSB butts up against the new OSB or is 12 inches close enough together to put the new OSB where there is no tung and grove option?
Not quite sure I understand the question, but if you are asking if you need blocking between the joists to help support the 3/4 inch OSB, the answer is no. But you may want to check a few things. 3/4 OSB is generally not stiff enough for tile. Don't know what you are planning to install, but tile is pretty common in a bathroom.
You may want to look at the Ditra (Schluter) website, they have very good information on minimum subfloor preparation for tile. In your case, 12 inches on center is closer than normal for joists, they are generally 16 inches OC, at least in the U.S., so you may be OK with only 3/4 inch OSB for tile.
Also, I have never used tongue and groove OSB, I am curious what type you have. Normally I use plywood, and screw it down to the underlayment, but then again I have two layers of subfloor, in my house the underlayment is 3/4 inch diagonally placed pine, and the overlayment is 5/8 inch plywood.