iGaming Ontario says operators generally let you deposit by online bank transfer, debit card or credit card, while the methods each site accepts vary (iGO FAQs). In practice that means Visa and Mastercard are the credit cards you are most likely to see in an Ontario casino cashier.
A deposit works like any online card payment: enter the card, confirm the amount, and pass any security check your issuer runs. Withdrawals are a separate question. A card that works for deposits may not be offered for payouts, so check the casino's withdrawal list before you deposit.
The important point is the cost. iGO warns that your bank may charge fees for gambling transactions and that every financial institution has its own rules, so call your issuer before using a credit card. Card terms decide how a gambling charge is treated and what it costs, and those terms differ from card to card.
Using borrowed money to gamble also makes it easier to lose track of spending. If you choose a credit card, set a deposit limit in your casino account as well; regulated Ontario sites must offer limit tools, and relaxing a limit only takes effect after a cooling-off period of at least 24 hours (AGCO).
Source: igamingontario.ca