iGaming Ontario lists online bank transfers alongside debit and credit cards as the methods Ontario operators generally accept (iGO FAQs). In a casino cashier this can appear under different names. Interac e-Transfer is the most familiar version in Canada, and the cashier will show which bank options that particular site supports.
The flow is similar across them: you choose the bank option, approve the payment through your bank, and the deposit is credited once the casino receives it. Bank options are also worth knowing about for withdrawals, because some deposit methods, such as prepaid codes, do not offer a way to receive payouts.
Timing and fees depend on the casino's processing and your bank, and they vary. iGO's advice is to contact your financial institution first, because every institution has different rules on igaming transactions, and some may charge fees. Regulated sites also keep a history of your deposits and withdrawals, which you can use if a transfer goes missing and you need to raise a dispute (AGCO).
Source: igamingontario.ca