I have been installing hardwood for customers for a few months and
things have been going well but seems I met my first challenge. When I
installed 4" wide 3/4" thick mahagony flooring there is excessive
height difference, maybe upto 1/10 of an inch in rare cases, between
adjoining boards.

I investigated and found this is due to the chipboard (3/4" OSB) floor
being very rough. Even though there is 1/5" thick plywood on top of the
OSB the problem persists. To correct the problem do I sand the OSB
floor or smooth it with floor leveling compound?
Or is there another solution?