Optimism utilizes [optimistic rollups](https://coinmarketcap.com/alexandria/article/optimistic-rollups-for-the-rest-of-us). Optimistic Rollups (ORs) can trustlessly record transactions on the second-layer blockchain and broadcast periodic merkle roots of the transactions to the first-layer blockchain. External validators verify these merkle roots, which delays withdrawing funds from layer-twos running on optimistic rollups (usually by one week).