About 20 people are currently living in the former car showroom on Iffley Road after moving in on New Year's Eve.
Leaseholder Midcounties Co-operative has said the squatters can stay until April, despite a judge granting it permission to evict them earlier.
But owner Oxford University's Wadham College said it requires the building back by 27 February.
This is so preparations for demolition can begin, the college said, as it intends to use the site for student housing.
Neo, a spokesman for the Iffley Open House campaign group, said it was "unacceptable" for the college to try to get the squatters out by February.
He added: "The weather will still be cold, I don't want people out on the streets."
The campaigners have asked that the building, which has been empty for two years, be officially designated as homeless housing.
Kevin Brown, group general manager at the Midcounties Co-operative, said in April the company has a legal obligation to hand the site back to Wadham College.
He added: "To ensure we're able to meet that requirement, we have been granted a repossession order but our preference is not to have to use it."
A Wadham College spokesperson said: "Whilst we have deep sympathy with the plight of homeless people, any solution must not delay the redevelopment of the building as homes for students.
"We will continue our discussions with all parties in the hope of resolving this amicably and are seeking help from local homeless support groups and charities to help find longer-term accommodation for these homeless people."
Squatters who turned an Oxford building into a homeless shelter have been given temporary permission to stay.