If you have an original style ignition switch, with the key in the cylinder, just push the cylinder into the switch body and turn it. It will click into place and stay there. There's a hole in the face of the cylinder where you stick a paper clip or small punch to depress a plunger and turn the cylinder with key in place to the left to release it.
Here's an old tech tip on the ignition switch for a 1937 truck.
LINK Your switch would be similar except for the mounting.
Here's a pic of an original style ignition switch and cylinder stolen from one of the vendor sites.
As far as an Opel ignition switch, I haven't a clue.
