installing hardwood flooring over multiple finishes

We need to instal a hardwood floor over an existing floor (previous hardwood removed) and over a newly added area. The problem is there is a slight difference in the level of the floor types. One area has a "lump" in the sheet of OSB sub-floor. What can we do to even out the floor to prepare it for the hardwood floor with out worry of the new hardwood popping up?