It's simple 3 steps approach
You will be calling API by posting the required parameters to get Payment URL for the transaction. This is the initial step of integration which is to create orders in Og Checkout platform.
Once Payment URL generated with the above step, redirect the customer to Payment URL and it may redirect to Actual Payment Gateway page or it may redirect to Og Single page which presents all the payment methods assigned to merchant. Here user /payer has to enter the Debit/Credit/Wallet details to get deducted the amount. Og Checkout platform will get updated by Payment gateway provider.
Soon after amount Captured/Not Captured status received from payment gateway provider, Og will redirect transaction Result to Merchant Call Back URL which passed in the request mentioned in Step 1. Depends upon the Result, Merchant portal can decide the receipt for the customer.