Optimism (OP) can be purchased via multiple crypto trading platforms, such as Binance, Coinbase, Gate.io, KuCoin, Huobi. First, compare the conditions offered by crypto exchange platforms, such as additional fees, customer support, account requirements, etc. Create an account and make a verified deposit. Now you can exchange your assets for OP in the way that you want - bank transfer, P2P, credit/debit card, and so on.