Question on If statement, should add when is not blank

Hi guys, 

Got a basic question, first Calculated Column "Celular vs Ingreso" checks the difference between 2 dates :

-Celular vs Ingreso = IF('KPI'[Fecha solicitud de ingreso]<'KPI'[Celular & Linea Resolucion],DATEDIFF('KPI'[Fecha solicitud de ingreso],'KPI'[Celular & Linea Resolucion],DAY),(-1)*DATEDIFF('KPI'[Celular & Linea Resolucion],'KPI'[Fecha solicitud de ingreso],DAY))

Second Column "-Celular" should return a status: if first "Celular vs Ingreso" column is greater than 0 days, should be "NO OK", if the column is less than 0 should be OK.

-Celular = IF('KPI'[-Celular vs Ingreso]>0,"NO OK", "OK")

The problem is there are some date columns that have no dates, so the value is null, so column has blank values, this is not a problem for column "Celular vs Ingreso", it returns no values for those null dates. But when advancing with the second calculated column "-Celular" the if statement is bringing me a OK as if there would be a zero value on it, and should stay blank

Already tryied with = IF(NOT(ISBLANK([-Celular vs Ingreso]>0,"NO OK", "OK"))) but i'm getting an error.

Any ideas on how to get this done?

Thanks & Regards!