Cutting hole for toilet pipe

hi

I'm doing some remodeling work, and my plumber just cut a hole for the toilet flange. right now, I only have 3/4" OSB subfloor, plus 1/2" plywood underlayment. I plan on putting down ditra and tile. the flange won't be installed until the end, right now the pipe is just a long stub-out that will be cut down.

The hole he cut is a little too big, in that the perimeter of the flange will be supported, but the screws won't have anything to bite into.

I have access from the basement. Can I cut a patch of plywood, and screw it to the bottom of the OSB? I figured he could use some longer flange screws to screw into the patch.

Thanks

Yes...he should know better, though.

I had a similar situation in where the top layer was cut to the size of the flange and the bottom layer a little smaller. I put expanding foam in around the pipe and trimmed it flush with the first layer. Then I put hydraulic cement in flush with the tile, then drilled that and put masonry anchors and stainless screws in to hold the flange in place.

This is what happened to me, the hole was cut out too big and the toilet screws doesn't bite into anything.

If I could go back, I would insist that part of the floor be redone until, it fits right. He fixed mine by laying 1/25 inch Louann board over it that fits nice but, the toilet leaks because the screws dangle right through the board. I can see it from the access panel. The louann board is now warping and the toilet rocks, and it leaks and leaks and leaks.

I simply gave up.

Take pictures.....this we gotta see.