Cases of coronavirus are continuing to rise in the area.
The Local Resilience Forum (LRF) - which covers Berkshire, Buckinghamshire and Oxfordshire - has made the decision following 'the increasing pressure on health and care services.'
The LRF is made up of local councils, the NHS and emergency services, and hopes the move will ease the pressure on local services.
In Oxfordshire, there's now almost 500 cases per 100,000 people.
LRF Chair, Deputy Chief Constable of Thames Valley Police Jason Hogg said: "The move to a Major Incident status enables the LRF to review and action additional measures as necessary as part of our multi-agency working that ensure we are able to respond effectively to support our local communities as a result of the pandemic."
"The aim of all our decisions is to support front line staff to be there for our local communities in the best possible way and this is what lies at the heart of this move. Declaring a Major Incident will help us meet continued and increasing demand across the LRF.
"If anyone needs urgent medical help, they should contact NHS 111 or via NHS 111 online and use A&E for emergencies. It is vital that people continue to follow government guidance on hands, face, space."