If for OT - probably easy for someone

This is probably very easy, but for some reason I am slightly having an issue.

I am needing to schedule labor services and I am attempting to budget the amount for services on the schedule. I am attaching an example of what I am attempting to do. Basically I am going to make multiple copies of this same little box for each day. For the attached workbook I want to fill in the number of guys I need for each department in B4:B7. I then want to fill in the times I want them scheduled to work. Fairly straight forward to this point.

The M4:M7 list the total hours per group per day. This particular place I am going charges time and a half after 8 hours. I am trying to make one column O4:O7 show me the total number of OT hours. Basically I want it to say if "M?" is over 8 fill in "N?" with the amount that it is over. So N4 would show up as 2 in the attached example. I then will make columns to determine the costs at each rate.

Is this something someone can help me with?

Thanks
Matt