How to get a flat subfloor

I want to put 3/4" engineered hardwood throughout my house. I have several subfloors (3/4" OSB), that are not flat. I'm using a nail/staple installation.One person told me to use Thinset; another said to use UltraPlan, and another said not to do anything (the rises are about 1/4" within 4" areas). What is the best method to get my subfloor flat? Cost is a factor, but I want the job done right.

I have a previously installed floor that is uneven due to the rises in the subfloor. What will happen if I leave this floor in place? The rises are about 1/4" in a 4 foot area.

Thank you for your help,

Dave

 

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