Is OSB stiffer in one direction

I would not tile for a temporary surface. I would get a sheet of sanded 3/4”. If you only need a 4x 6 or 4x 8 lay the sheet down and cut the carpet at the edge. Remove the cutout carpet and tack strip. Put plywood in the hole and use a few deck screws to hold the plywood in place. I might get some carpet threshold and lock the carpet down or use some screws and washers to hold the carpet in place.

The plywood will hold up for a few years and be easy to remove. Tile will be more expensive take longer to put down after that all the work to remove and dispose of the debris.

If you want a surface that will last a Formica sheet on top of the plywood will last a decade or more. And when pulled up could be used someplace else when done.

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.

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.