Students caught breaking coronavirus laws could be suspended or expelled in the most serious cases, a university's vice-chancellor has warned.
Prof Alistair Fitt said a "minority" at Oxford Brookes University had joined large social gatherings outside halls.
He said this was "unacceptable" and "firm action" would be taken against those who broke regulations.
The university said on Wednesday it was dealing with a "small but growing number of confirmed cases".
An "outbreak" developed last week in private accommodation off the main Headington campus.
Prof Fitt said the rise in Covid-19 cases among students was "extremely disappointing".
He said he understood many people would be "alarmed" to see images of large crowds gathered outside one of the university's halls of residence earlier this week.
"Where we deem students to have contravened our conduct regulations, we will take appropriate, robust action, which may include suspension or, in the most serious cases, expulsion," he said.
Prof Fitt said the university had "communicated extensively with students to ensure that they understand their responsibilities".
The vice-chancellor said he had spoken with Thames Valley Police to put measures in place to prevent further gatherings.
Oxford is once again on amber alert, with the current the number of cases of coronavirus in the city standing at 25.6 per 100,000 of the population.
There was an amber alert last month but figures came down again.
An amber alert happens when cases rise above 25 per 100,000 of the population.