The ERC-20 address of the legitimate Optimism Token is 0x4200000000000000000000000000000000000042 on Optimism. You can check your balance by going here and updating the filter address to your address.
Note: If you receive a direct message (e-mail, Twitter DM, etc.) from an account claiming to represent Optimism and offering to sell you tokens, especially at a discount or earlier than when they are generally available, it’s a scam. Please block, report, and ignore.