Managed to get from H2OS11 to OOS11.Not sure to be honest, I know it won't allow a local upgrade to a previous version, it may do if they're both Open Betas, I would suggest removing your security lock first (i.e. Change from pin/pattern to None) to save any encryption issues, you should be able to place it back on once you install OxygenOS
Used local file upgrade and rolled back to a rollback image I found in the FAQ thread here.
Let updates update
Did local file upgrade to OOS11
Much happier on the Oxygen side of town. Items that didnt work for me in Hydrogen do now and no Chinese bloatwear