Contracts have been exchanged on the property, which has been owned and operated for the last six years by the de Savary family, and are due to complete on 1 August.
The Old Swan, an inn with 15 guest rooms and the adjoining 44-bedroom Minster Mill, marks Brownword Hotels' first move into Oxfordshire.
Brownsword Hotels owner Andrew Brownsword said all Old Swan & Minster Mill staff would be retained and the business would continue to operate 'as usual for the immediate future'.
He said: “We’re delighted to have acquired the Old Swan & Minister Mill, adding an Oxfordshire property to our collection of beautiful country house hotels and our city centre ABode group.
"The hotel has a great offering for both leisure guests and the events market, and we look forward to working with its team to build its reputation still further”.
Following its purchase by the de Savary family in 2010, the Old Swan & Minster Mill received 'significant investment' to turn it into a five-star inn.
Lana de Savary, who also owns the Cary Arms in Devon, said: "The Old Swan & Minster Mill is the most lovely place. Our loyal, passionate and enthusiastic staff, many of whom have been with us for the past six years, are the reason our thousands of guests have happy memories of their stay.
"I am confident that Andrew Brownsword Hotels will continue the attention to personal detail and further enhance the hundreds of years of history attached to the inn. I wish all the staff and the Brownsword family a very happy and continued success”.
The de Savary family's main business objective is to acquire under-performing hotels with the intention of investing in them and turning them around. Previous properties include Skibo Castle, Bovey Castle and Stapleford Park.
On joining Brownsword Hotels The Old Swan & Minster Mill will become part of a group of privately owned hotels which includes Gidleigh Park, The Bath Priory, Amberley Castle and The Slaughters Manor House.