OP Mainnet (Optimism) is an optimistic rollup that batches transactions to Ethereum and relies on fraud proofs submitted during a seven‑day challenge window. The Bedrock upgrade in 2023 cut L1 calldata costs by 40 % and introduced multi‑proof modularity. ‘RetroPGF’ rounds redistribute protocol revenue to public‑goods builders, governed by the OP token‑holder Collective.