Your OR functions will always return TRUE if a non-zero value is entered in any of the cells being checked when used in their current format.
I propose the following modification:
=IF(OR(I5>7,N5>7,J5>7,O5>7),"Greater than 7 MPa",IF(OR(I5<4,N5<4,J5<4,O5<4),"Less than 4 MPa",""))