More than four-fifths of would-be Oxford students are being asked to sit extra aptitude tests to win a place at the prestigious university, and the numbers have soared in recent years.
"Significant" increases in applications have meant the institution now has to use additional measures besides looking at exam results to identify the best candidates, according to Oxford's undergraduate admissions director.
Mike Nicholson said the university is using the tests as a "sifting process" to give tutors additional information before deciding who to interview for places.
In the coming year, around 85% of applicants will take "some form of aptitude test", he said, which are used by 70% of subjects offered by Oxford.
This has gone up from just under two-thirds of candidates, between 60% and 65%, three years ago, and could rise further.
"It's predominantly been driven by the significant increase in applications that we've seen in the last five years," Mr Nicholson said.
In a subject such as economics and management, there are now around 1,300 applicants for 96 or 97 places, whereas 10 years ago there were half as many applications for the same number of spots.
Mr Nicholson admitted Oxford is not able to interview every applicant, with around 65% being offered one - about 24,000 interviews in total.
He insisted the tests are "not the only measure" Oxford uses to identify students, as they are used to check candidates' aptitude and suitability for a subject, rather than just what they already know or have learnt to pass A-levels.
The tests are tailored to different subjects, to check types of skills, such as data interpretation, mathematical knowledge and the ability to write "cogent, focused essays" within a time limit.
Mr Nicholson revealed that it is not just traditionally over-subscribed departments, such as medicine, that are using the tests, and he predicts the numbers using them could rise if more departments see further increases in applicants.
This year, Oxford had around 17,200 applications for about 3,200 places.