The purpose of the tech titan doing this is to change direction from OpenAI’s technology, which the product uses to support itself.
According to Verdict, the move is "intended to reduce costs and improve performance for enterprise users".
It comes as Microsoft – which has backed OpenAI – wants to reduce "the reliance on the AI startup".
An agreement between the company and OpenAI allows Microsoft to customise OpenAI models.
A Windows maker is quoted by Verdict as saying: “We incorporate various models from OpenAI and Microsoft depending on the product and experience.”
The publication added that "OpenAI remains a partner for frontier models, the most advanced AI models available".
The American company is creating its own AI models to improve the speed and quality of 365 Copilot – with the hope of making it cheaper to run.
Microsoft's head of AI, Mustafa Suleyman, previously said AI assistants with “really good long-term memory” are about a year away.
He added to BBC News: "I think we’re moving to a fundamentally new age where there will be ever-present, persistent, very capable co-pilot companions in your everyday life."