"We are just about to reach an agreement," said a spokesman for IG Metall on Thursday.
The negotiations, which started in June and were supposed to complete by the end of October, could be concluded on Thursday but may need an extra day, according to the union spokesman.
Opel could not be reached immediately for comment.
The deal is expected to extend job guarantees by another two years to 2016 for all four German plants, which together employ around 20,000 workers.
At the start of 2017 at the latest, however, assembly lines at the Bochum car plant will cease running and only some components and warehousing jobs will remain under the deal.
A union source involved in the negotiations told Reuters labour leaders were not willing to agree on further wage concessions and job cuts in Bochum.
Workers fear GM wants to "hollow out" Opel, where labour has had certain rights under German law ever since it became a joint stock company in 2010.
Operating assets outside of Germany once owned by the company were being successively regrouped directly under parent GM, triggering union suspicions that Detroit wanted to undermine union influence in product, investment and strategy decisions.
Unions want to avoid further decisions such as the one taken a year ago to build the Opel Antara SUV in South Korea, taken by parent GM above the heads of Opel management and its supervisory board, in which unions have a seat at the table.
GM's second-largest brand behind only Chevrolet, Opel has lost billions in recent years as Europe's car market has plumbed near 20 year lows, despite repeated bouts of job cuts that included closing a car plant in Belgium at the end of 2010.
(Editing by David Holmes)
By Jan Schwartz and Christiaan Hetzner