This is only one section, but it's the worst one. The rest of them just had random sistered joists for no apparent reason.
I added the blocking on the right side (and the tile that I should have left until the end, but I was trying to be efficient and it's only a little in the way), but the OSB is all thanks to the previous owner. They sistered 2x4s to the joists on either side, then added a piece of OSB on top. Not just any OSB, though, a piece that they had cut into--I think for the toilet--then decided not to use, probably because they screwed up the cut. So my first question is, is this a legit way of adding support? I just did the standard 2x4s (or, in the case of a joist with a heating pipe in the way, 1x4s) under the cut edges and toenailed in place. I told my dad about this and he just said "yuuuuup." No one will ever describe him as loquacious.
My next question is, why? Why not just toenail in the 2x4s you screwed to the joists? It's OK to throw away construction materials when you screw them up, I promise. I've done it myself. I cut a 2x4 too long, then too short when I tried to fix it. You know where it is? The trash. I could hoard it and hope I need a footlong piece of 2x4 sometime, but I think I'll risk tossing it.
And finally, why did I leave it? Because I did. I'm sorry. I wanted to take it out. But there were screws everywhere, even hidden--sparks!--and it seemed pretty stable. It held my weight so I don't think it's going anywhere.
Oh, bonus question. Why did the previous owner leave a huge gap around the toilet pipe? Why not cut the subfloor in half and go in from either side. Inquiring minds want to know.
This subfloor has been a huge pain in the ass, just like the tile floor before it. I'm not even a little bit surprised. I hope there are no more surprises; I'd like to finish this up now. I've been in there with a handsaw and a chisel getting into the corners where the circular saw won't fit. The rotozip doesn't like this floor, so it's not super useful. I have one more corner to clean up before I can put the last 1/2" patch in, then we move on to the 3/4" sheets, then the primer, then the tile, then the grout, then the grout sealer. I can see the light at the end of the tunnel.