The Guardian Media Group has owned the Observer since 1993, with around 70 people working on the newspaper.
Staff had already been told that if the sale went ahead, they could take voluntary redundancy on enhanced terms or transfer to Tortoise on their existing contracts.
Freelancers were also told their contracts would be extended to September 2025, and then renegotiated.
Its circulation had been steadily falling until 2021, when it stopped publishing audited figures. At that point it was selling around 136,000 copies a week.
Anna Bateson, chief executive of Guardian Media Group, said: "This investment will preserve the Observer's 233-year legacy and protect the paper's future, ensuring it can continue producing exceptional liberal journalism, online and in print, for years to come. Underpinning it all will be a continued commitment to promoting a free press and maintaining editorial independence.
"The deal also supports the long-term success of the Guardian, building on our growth globally and across digital, as we continue to put readers at the heart of our outstanding journalism."