What GM car lines have been discontinued besides Opal and Oldsmobile?

As has been mentioned, Opel is still around.

Oldsmobile was let go a few years ago. I think the next one back in time was the 1940 LaSalle, a companion make to Cadillac. A companion to Olds, Viking, was around for 2 years or so and died in 1931 or so. Pontiac was a companion make for Oakland, and the latter died in the early 1930s. Buick's Lafayette died too around that time or just before the stock market crash.

There were other cars that GM bought that were merged before the 1930s, such as the two truck companies that became GMC. Maxwell was basically a pre-Chrysler car and had nothing to do with GM.

There may be some European brands that don't exist under GM's wing anymore . . . Bedford was a truck company that was like Vauxhall's older brother, but I don't know if they're still around or not.

Opel was never discontinued. It's still a major brand. They are not currently marketing the Opel brand in the US. They are rebadging the Opel cars that we get.

Anyway, during the depression, they introduced "Junior" marques Lasalle, Marquette, and Viking (and Pontiac). Pontiac was the Junior Oakland.

so they discontinued Oakland, Viking, and Lasalle. viking was not much of a seller, but Lasalle was. Lasalles from the 1930's are actually pretty numerous.

Oakland and Maxwell

Opel is still around.

Very soon, Saturn and Hummer will go.