Keith Strangwood, chairman of Keep the Horton General, said the group is getting numerous reports of patients being sent home because the operating list has been cancelled.
"It's that time of year again when the closure of so many beds means the hospitals are having to use what beds they have for the 'winter pressures' - often people with complications from flu or falls," he said.
"In the last week alone we have heard from a man in his 70s who had reached the JR hospital, Oxford for a very necessary operation, only to be told they had tried to call his mobile to say his procedure had been put off. The ward said they had tried to call him but he had not received a call. Many areas of Banburyshire have very bad coverage.
"I have had another man who has had his op cancelled three times. On two occasions he didn't know until he reached the JR and the third time, it was by phone, moments before he left for Oxford.
"This isn't just a question of the anxiety, stress, travel costs and sometimes dangerous delay for the patients, it is also about the ludicrous waste of consultants' time. In these cases, the consultant, anaesthetists and theatre staff were left with an empty list and empty theatre at huge cost to the hospital trust."
Mr Strangwood said the problem was especially bad for patients from Banburyshire since they have to allow at least two hours to get through the traffic to the JR.
"We have always said that closing 45 of the Horton's beds and removing our emergency surgery was a bad thing. Now, once again, we have a shortage of beds having dire results.
"There is still bad bed blocking in our hospitals because there are not enough nursing and care home beds and patients who might be able to go on to care elsewhere but cannot. They talk about 'care at home' replacing the closed hospital beds but some people are simply not well enough to go back to their own homes.
"The government needs to stop building endless estates around our towns and villages and concentrate more on building adapted, sheltered accommodation and care homes to cater for the ageing population."
A spokesman for the Oxford University Hospitals Trust (OUH) said: "Regrettably, some elective procedures (planned operations) have been postponed to allow us to prioritise the care of our sickest patients. Our trust has not declared Opel level 4 (the highest alert level) this winter."
Sam Foster, Chief Nursing Officer at the Trust, said: “Winter is always a time of high pressure for emergency departments (ED) nationally. The increased acuity of patients coming to our emergency departments at the JR and the Horton is reflected in the increased number of ED patients being admitted to hospitals for treatment. This is line with the national picture.
"We are monitoring this situation daily to make sure that we deploy staff to provide support where it’s most needed and to provide care in the safest way possible."
Ms Foster urged people to only go to A&E in an emergency.
"GPs, pharmacists and minor injuries units can provide support for several conditions and injuries. NHS 111 can also put people in touch with medical professionals and signpost people to the most appropriate support," she said.
“We are working hard with our health and social care system colleagues to deliver a ‘home first’ approach, and help patients leave hospital and receive care closer to home – allowing us to care for patients who genuinely need to be in hospital.”