The minor leagues are undergoing major changes in 2021, and you can click here to read all about it. The upper minors (Triple-A and Double-A) are staying mostly the same, but the lower minors are getting a significant shakeup.
The short version: The various levels of Single-A ball are shuffling around, with some leagues moving up or down the hierarchal ladder, and the short-season Low-A level is eliminated entirely in favor of a new Draft Scouting League that focuses on that year’s draft class.
For the Oakland A’s, this means some changes in affiliation.