OWL, which has 12 staff, runs user-led community projects in Sawston, Cambridgeshire, providing work, education and activities for people with disabilities.
It will become part of the Papworth Trust, which operates in eastern England and employs about 300 people, but will retain its name and branding and continue to run services at its existing premises.
A spokesman for the Papworth Trust said: "OWL has a great deal of expertise in social enterprises for disabled people and we can learn a lot from that. It has some really good projects that we want to use and develop so we can achieve our aim of helping more people."
The spokesman said he could not guarantee there would be no job losses but said it would be "very unlikely".
In a statement, Kevin McMullen, chairman of OWL, said: "This is a fantastic opportunity for two of Cambridgeshire’s best-known charities to come together and offer even more choice to disabled people."
The deal is expected to be completed in the autumn and the organisations will start working together formally in September.