The tub that this connects to drains slowly. I've snaked it from the tub and the overflow but not found anything. I think I need to open this cleanout but it hasn't been opened in at least 18 years (before I bought the house). I'm thinking apply penetrating oil, wait, use a 1.5" box wrench.
I took the advice to use a second wrench to hold back the cleanout but the cleanout is tight against a double 2x4 so I can't get the pipe wrench around the body of the cleanout. I put it over the exit hoping that was good enough.
But the 15-degree offset box wrench I have won't sit securely on the top of the cleanout. The "nut" is 3-4" below the floor level so the wrench head is still not level with the top. I don't think there is room to put the box wrench completely in the hole. It might work if the wrench wasn't offset. And I might be able to keep the box wrench down with a third hand but I don't have room for a second body in the closet where I have to work.