Students from Oxford Brookes University made up 70% of coronavirus cases in the city after they doubled in a week.
The university said out of 68 cases confirmed in the seven days up to 25 September, 47 were students.
It comes as Brookes issued 150 fixed penalty notices after the rise in cases was linked to social gatherings at halls of residences.
Oxford University is asking students to sign a "responsibility agreement" ahead of Freshers Week starting on Sunday.
The institution, whose colleges host about 24,000 students, said the contract outlined "new habits and adjustments" required to "enable academic and social activity to take place as safely as possible".
Health chiefs in the county are worried that with the current rise in cases, the city is on course to move into the "red alert level".
Since the outbreak at Brookes, the university has said smaller groups of teaching are being organised on campus and all people are required to maintain social distancing and wear face coverings indoors.
In a statement, Brookes said face-to-face lectures had been "scaled back" and services were being delivered online "wherever possible".
The university said it had also built a testing centre at its Headington-based campus which was also open to members of the community.
Vice-chancellor Prof Alistair Fitt previously said students caught breaking coronavirus laws could be suspended or expelled in the most serious cases after footage showed large gatherings outside halls of residences.
Ansaf Azhar, Oxfordshire County Council's director for public health, has warned if cases continue to rise "at the rate they have been doing, then we may have no choice" but to enforce stricter restrictions.