In 1913–14, and under threat of competition from William Morris and Frank Gray, the tram company replaced its trams and horse buses with Daimler motor buses.

In 1921, the company was renamed City of Oxford Motor Services Limited (COMS).
It continued to expand its operations into the surrounding countryside.
From the 1930s, COMS was controlled by British Electric Traction, with the Great Western Railway having a minority shareholding.
The fleet livery was red with maroon and pale green relief.
Most of its buses were built on AEC chassis and running gear.
Numerous former COMS buses have been preserved, including a large and notable collection at the Oxford Bus Museum in Long Hanborough in Oxfordshire.

In 1969 COMS became a subsidiary of the National Bus Company.
Greater integration of city and country services began.
In 1971, the Oxford – London coach operator South Midland, which had been controlled by the neighbouring Thames Valley Traction company, was transferred to COMS and the fleet name for the entire operation became Oxford South Midland.

An acute problem for the operator was the competition for staff with Morris Motors, whose Cowley factory was near the Oxford garage.
One response was to move to one person operation of buses in the 1970s.

After some trials, the Oxford company established the city's pioneering park and ride bus services in 1978.
In the same year, the London express service began, using its present route.

In 1983, COMS was split into separate Oxford and South Midland units.
The Oxford Bus Company was allocated the Oxford city services and the London routes, and South Midland was allocated the remainder of the network.
Both companies were subject to management buyouts.
The South Midland company was soon resold to Thames Transit (later Stagecoach South Midlands), which introduced minibus competition.
Oxford Bus Company tried to counter this with minibuses under the Oxford City Nipper brand name.

In 1990, the Oxford Bus Company acquired the High Wycombe operations of the Bee Line, and ran them under the Wycombe Bus brand name.
In March 1994, the Go-Ahead Group bought Oxford Bus Company.
In 2000, Go-Ahead sold the High Wycombe operation to Arriva.
The company's long-established main depot in Cowley Road, Oxford was closed in 2004, replaced by a new depot opened in Watlington Road.

In July 2009, Oxford Bus Company took over Oxford Brookes University's BrookesBus contract.

Go-Ahead bought Thames Travel in 2011 and Carousel Buses in 2012.
They now share Oxford Bus Company management but retain their separate identities.

In October 2019, it was announced that the X90 service between Oxford and London would be withdrawn from 4 January 2020, due to a 35% fall in passenger numbers since 2015 causing the route to be unprofitable.
In 1913–14, and under threat of competition from William Morris and Frank Gray, the tram company replaced its trams and horse buses with Daimler motor buses.
In 1921, the company was renamed City of Oxford Motor Services Limited (COMS). It continued to expand its operations into the surrounding countryside. From the 1930s, COMS was controlled by British Electric Traction, with the Great Western Railway having a minority shareholding. The fleet livery was red with maroon and pale green relief. Most of its buses were built on AEC chassis and running gear. Numerous former COMS buses have been preserved, including a large and notable collection at the Oxford Bus Museum in Long Hanborough in Oxfordshire.
In 1969 COMS became a subsidiary of the National Bus Company. Greater integration of city and country services began. In 1971, the Oxford – London coach operator South Midland, which had been controlled by the neighbouring Thames Valley Traction company, was transferred to COMS and the fleet name for the entire operation became Oxford South Midland.
An acute problem for the operator was the competition for staff with Morris Motors, whose Cowley factory was near the Oxford garage. One response was to move to one person operation of buses in the 1970s.
After some trials, the Oxford company established the city's pioneering park and ride bus services in 1978. In the same year, the London express service began, using its present route.
In 1983, COMS was split into separate Oxford and South Midland units. The Oxford Bus Company was allocated the Oxford city services and the London routes, and South Midland was allocated the remainder of the network. Both companies were subject to management buyouts. The South Midland company was soon resold to Thames Transit (later Stagecoach South Midlands), which introduced minibus competition. Oxford Bus Company tried to counter this with minibuses under the Oxford City Nipper brand name.
In 1990, the Oxford Bus Company acquired the High Wycombe operations of the Bee Line, and ran them under the Wycombe Bus brand name. In March 1994, the Go-Ahead Group bought Oxford Bus Company. In 2000, Go-Ahead sold the High Wycombe operation to Arriva. The company's long-established main depot in Cowley Road, Oxford was closed in 2004, replaced by a new depot opened in Watlington Road.
In July 2009, Oxford Bus Company took over Oxford Brookes University's BrookesBus contract.
Go-Ahead bought Thames Travel in 2011 and Carousel Buses in 2012. They now share Oxford Bus Company management but retain their separate identities.
In October 2019, it was announced that the X90 service between Oxford and London would be withdrawn from 4 January 2020, due to a 35% fall in passenger numbers since 2015 causing the route to be unprofitable.
Oxford Bus Company