Leveling Uneven Subfloor

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BACKGROUND:
Doing a kitchen remodel on our own, trying to open up existing kitchen into formal dinning room. As soon as the non-load bearing wall is gone, I have this small issue as shown in the picture below…
The kitchen flooring on the right is the original subfloor with three sheets of 1/4" plywood glued and screwed down (what the heck?!?!) for the top sheet of linoleum. After tearing out the carpet in the formal dinning room, there is a 3/4" gap that gradually levels off.

QUESTION:
What is the best way of filling this rather large, un-level area? It spans about 4' x 13'. I foolishly already bought enough 3/4" OSB to cover the whole area, but now I'm second guessing myself! Seems like too big of a surface area to use leveling compound…

SIDENOTE:
I'm guessing some guys are probably going to say something about making sure the floor is properly supported, check my joists, etc. But I've been reassured there are no structural issues below.

not level

Best Answer

Screw down 3/4" plywood or cement board for the deep area & 1/4" for the less deep. Then, use very much less floor leveler for a final fill-in. Make sure you account for the flooring, so the 3/4" might need to be 1/2" & the 1/4" may need to be shortened. Do not use shims & create voids.