OSB is for subfloor and MDF is for underlayment. If you can't get 5/8" MDF, then consider getting 1/2" MDF + 1/8" hardboard. You can use OSB but I would personally go sanded plywood. Same thickness and shim it up using chipboard or similar so that its at the same height.

While the 5% difference is barely noticeable, for LVP and similar flooring options, it will be a pain to install and will crack over time.