In south GA, hybrid turnips like T-raptor and Winfred have been a good companion crop to my spring oats planted in September. The small seeded brassicas really like a prepared seedbed though. If you are overseeding, considering grazing radish, although cows will choose many other brassicas over that one. It's larger seed is more likely to come up in a competitive sod. Brassicas are extremely wet, so you have to slowly introduce the cattle to them, and ideally strip graze and provide hay. You can stomp them out if you overgraze.