Question about Stadler Form Antique lime cartridge

No, Oskar Big does not have a removable water tank where this cartridge would be mounted.
Since Oskar Big is also a vaporiser and not a nebuliser, an anti-limescale cartridge should not be necessary.

I don't know Oskar big, so I can't answer your question. There should be a hint in the instructions for use (Oskar big and cartridge) -> do some research!