IF OR ISERROR problem

Given the explicit coercion taking place as a result of the subtraction you need to test the Error first / separately:

=IF(ISERROR(O2-J2),"",IF(O2<=J2,"",O2-J2))

we don't know how O2:J2 are being populated but you might find you could get away with:

=IF(OR(COUNT(O2:J2)<2,O2<=J2),"",O2-J2)

but the former is more "robust"