Full time versus part time hours

Overtime for full time employees (40 hour workweek) in Michigan kicks in after the 40 hours.  So I am unsure about the 30 or so hours differential.  If the issue concerns whether it is permissible, it is permissible depending on what the employer's needs are, unless of course the employer is simply doing so to avoid paying the wages required for full time employees while avoiding having to pay benefits for part time employees.