This is purely subjective. I've seen (different) people on here report that the actual OAT was more difficult than the OAT Achiever, and others report vice-versa.
My experience was that the questions on the OAT Achiever were much more difficult/challenging than the questions I experienced on the actual OAT. This could be attributed, however, to me "knowing" the answers to the questions asked on each...
Definitely more difficult (except for RCT) than the OAT from my experience. But if you could get used to those challenging questions from OAT Achiever, it would mean fantastic scores for the actual OAT exam... 👍👍👍