The blaze started in a farm outbuilding on Ickford Road just after 3pm.
The barn contained mixed waste including shredded plastics, fibreboard and woodchip.
Crews from Aylesbury, Waddesdon, Thame, Wheatley and Oxford attended, along with four officers.
Firefighters used two main jets.
Residents in Shabbington and Long Crendon were yesterday advised to stay indoors and keep windows shut because of the plume of smoke.
Bucks Fire and Rescue have been liaising with the Environment Agency to minimise the environmental impact of the blaze.
One crew remains at the scene this morning.