The German government supported Magna International's efforts to buy a majority stake, providing $6.5 billion in state-backed guarantees to be spent on restructuring.
Al Jazeera's Alan Fisher said the deal "was about the future of 50,000 jobs and the future of GM in Europe".
"It was also a political victory for Angela Merkel," he reported.
"Germany wanted the Magna deal, believing it would secure more German jobs and with an election at the end of the month, this was a politically sensitive decision."
The package deal was cautiously welcomed by the workers.
One worker, who remained unnamed, told Al Jazeera: "It will be the best solution for us. They are going to safe our jobs at least, I hope so."
Negotiations are expected to continue.
Details of the deal, including any feared job cuts, have not been made public.
Throughout the negotiations, GM indicated it wanted to keep a stake in Opel, which is based in the west German town of Russelsheim.
It is also understood to have imposed conditions on the sale to Magna, a car parts manufacturer.