Fix 40 kg TV to plasterboard and OSB wall

With 18 mm OSB attached to steel studs and drywall over that the wall is very strong. I don't think it would be necessary to screw the bracket into the studs. Four appropriate screws into the OSB should be sufficient with a safety factor, and no more than six would possibly be needed. This assumes that the bracket is fixed and holds the TV close to the wall.

Assuming that "plasterboard on OSB" means that the studs were first fully covered with OSB, then that was dressed up with plasterboard for a nicer finish, you have a very sturdy wall! I just build a storage shed using 3/4" (~18mm) plywood as the floor. I expect it to hold 100s of kilos at the contact points for each of the shelving units.

If this is how your wall was built, you've got the added advantage of being able to put the TV anywhere you want because with the proper bolts, you don't really even need to worry about hitting a stud, that 18mm OSB should hold your TV without a second thought. Of course, hitting a stud is an added bonus, and will take the proper sheet-metal screws to get a good grip into the steel studs.

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If you're referring to OSB on the outside of the building and drywall on the inside, then this advice goes out the window, use sheet-metal screws into the studs as Ecnerwal suggested. In either case, you have nothing to worry about.

Use proper sheet-metal screws into the studs. "Self-drilling" screws will save a step, or you can pre-drill the proper size hole for the screws.

Your worry is unrealistic - steel studs are quite robust.