(emphasis added)
@MYOS1634, UCAS is explicit that you have to report all ‘qualifications’- and APs are qualifications. I am surprised to hear that you know ‘countless’ applicants who have left off unflattering scores. I don’t know of any place that says ‘this only applies to UK applicants’ or ‘US students are exempt from this rule’, and have heard directly from Oxford people that they want to see all test scores.
Would an applicant leaving out results get caught? like any form of cheating, probably not- but I wouldn’t recommend it, on many levels.