Remember, oats/wheat/rye are most palatable during their first 90 days of growth. Therefore, it's best to plant them in early fall. They are hardy plants and can withstand frosts within the first 30-45 days of growth. A hard freeze will retard or even kill them if they are less than 20 days old.
Here on the northern Cumberland Plateau, we usually do not get a hard freeze until November. That's why I try to plant after September 15th through October 10th. That way the plants will have time to grow some before it freezes regularly.