Hello, I had to replace the subfloor in my bathroom around the toilet not the whole thing just a cut out piece. Do I have to fill the gaps between my butted up pieces? If so with what? Secondly I also have to lay a partial piece of 3/4 on top of my base do I need to lay anything down on top of the base before laying my 3/4 osb. I plan to top all of this off with a 1/4 piece of luan and then use filler before laying my sticky back tile.

Don't fill the gaps, they are needed for expansion. Might skip the luan too, if you go with plywood instead. Big discussion on that recently, forget where. No real conclusion. The sticky tile may be a little more touchy than vinyl. Could be a good argument for luan. Just make sure everything is screwed or stapled down good and smooth.