53% of Oxford's first year students were from state schools last year
Oxford University's head of admissions has said there is a "finite pool" of state school sixth-formers it can draw on to widen its intake.
Mike Nicholson said most of the candidates qualified to fill Oxford's 3,100 undergraduate places every year already applied.
More needed to be done, he said, to widen the horizons of younger pupils and increase the talent pool.
Oxford and Cambridge both missed their latest targets on state school pupils.
Both universities say they are working hard to increase the number of state school and working class pupils they accept.
Mr Nicholson, Oxford University's director of admissions, is speaking about widening participation at the party conferences.
He said that currently only around 28,000 teenagers achieved three A grades in their A-levels each year.
Around 11,000 applied to Oxford and a similar number to Cambridge.
Oxford wanted to increase the "potential pool of candidates" applying to the university and was committed to helping more disadvantaged teenagers reach higher education in general, he said.
It was involved in outreach programmes which would get more disadvantaged youngsters to "apply to all universities, not just Oxford".
The university targets children as young as 11 in its programmes, and their parents.
"We need to reach pupils at a very young age and make them aware of something they could study they had not thought of," Mr Nicholson said.
"It might not be a subject they can study in school, but there are other places they can access it."
"We need partnerships between all parties, teachers, government and the pupils themselves," he added.
Reaching pupils at a younger age could mean that more progress "to the point where they get three grade As at A-level", which make them eligible to apply to such institutions as Oxford.
At Oxford, some 53% of first year undergraduates were from state schools, while at Cambridge the figure was slightly higher at 57.6% in 2006-07.
Independent schools educate about 7% of pupils in the UK.
But the Sutton Trust charity, which strives to widen access to the best institutions, says that in 2006, 15% of the entries for A-levels and equivalent qualifications were from the independent sector - as were 35% of the students who achieved three or more A grades.
Figures out in the summer showed more undergraduates were coming from areas which do not normally send many students to university.