Optimistic Ethereum (OΞ) is a Layer 2 network that sits on top of the Ethereum Layer 1 network. Most ERC-20 tokens were created as a Layer 1 token. In order for that token to be available on Layer 2, someone (e.g. the development team) must bridge that token from Layer 1 to Layer 2.
You can find the most up to date reference in the OΞ token list. This list will continue to grow over time.