Can my employer not pay overtime since we are salary employees and work seasonal but receive year?

... round pay? He says we will only receive o.t. pay at the end of the year if the overtime hours worked are more than the offset of the 0 hour work weeks. Ex we have 120 OT hours by the end of the season that were not paid but after the first 4 1/2 weeks of the off-season those hours are offset by 4 1/2 weeks of 0 hours worked. Is this legal?