oneip1,
Oxford's pass rate is around 60%, but it obviously varies from Skills Assessment to Skills Assessment: on my assessment, pass rate was 20% but I know of someone whose assessment pass rate was 100% - it's all up to you, but it shouldn't be too tricky!
Have a look at the EPST website (
http://www.epst.com/) which runs the COMPASS test at Oxford, as they have a test you can try which is fairly similar to COMPASS!