The contract, announced today, was tendered in August last year in six lots with a total estimated worth of nearly £50 million.
Companies will start between June and August and the contract can be extended for a period of three more years.
OCS won three lots, worth up to a maximum of £3.8m per year in the contract notice, to maintain Hyde Park, Kensington Gardens, St James’s Park, The Green Park, Grosvenor Square and Brompton Cemetery.
The Landscape Group won the contract for The Regent’s Park and Primrose Hill, worth up to around £1.4m per year in the tender document, and the outer London parks, Greenwich Park, Richmond Park, Bushy Park and the Longford River, will be maintained by Turfsoil, which the contract notice estmated at £2m per year.
A spokesman said The Royal Parks would not reveal the final bid values for commercial reasons.
Deputy chief executive and director of parks Colin Buttery said: "The eight Royal Parks are among the most beautiful and popular open spaces in the world, which is in no small part down to our highly skilled and dedicated landscape maintenance staff.
"We have undertaken a robust and competitive procurement exercise and the successful contractors have demonstrated that they can offer exceptional quality and value for money.
"We look forward to working in partnership with our landscape maintenance contractors in order to keep the parks looking at their best."
Existing contract staff are covered by TUPE regulations, which ensure that terms and conditions are protected if a post is transferred to a new employer.
A significant number of the parks gardeners have been in post for many years – some more than 40 years.
The Royal Parks are managed by park managers directly employed by The Royal Parks, a government agency, while the work is undertaken by contractors.
The contracts cover all aspects of grounds maintenance and gardening in the parks, litter picking, refuse collection, road and path sweeping, cleansing, gritting, playground inspection and grave digging at Brompton Cemetery.
As part of the contract the chosen companies must achieve a minimum recycling rate for litter of 40 per cent at each park by the end of the second year of the contract.