Four residents needed hospital treatment after emergency services were alerted to a chemical smell in a block of flats.
Firefighters, police officers and paramedics were called to Greenhill Court in Lodge Close at 9pm on Tuesday.
Fire crews wore breathing apparatus and specialist detection equipment to try to identify the unknown chemicals involved.
Oxfordshire Fire and Rescue Service sent four engines from stations in Banbury, Deddington and Oxford.
South Central Ambulance Service and Thames Valley Police also attended.