It's important to plant oats early. But research at Iowa State College shows that you can generally afford to wait until you can prepare a good seedbed. In central Iowa, you're "safe" until about the middle of April.
Wiggans, S. C. and Frey, K. J.
"Your Oat Seeding - How Early - What Rate?,"
Iowa Farm Science: Vol. 11
, Article 7.
Available at: https://lib.dr.iastate.edu/farmscience/vol11/iss9/7