How pay opening balance for supplier?

It's a pleasure to have you in the Community today, @nurimran. Let's work this out so you can appropriately pay your supplier in QuickBooks Online (QBO).

Before we do so, may I ask what account you chose to use when you entered the transaction using the expense method?

In QBO, when paying a supplier, you'll only need to create a bill and then use the bill payment feature to finish the transaction. But since you've already entered it as an expense, you can proceed to create a bill and then link the expense so that when you pay the bill, It'll also clear the expense out. I'll gladly input the steps below. To begin, here's how:
 

1.  Access your QuickBooks Online company.
2. On the left navigational bar, click the +New button.
3. Select Bill, and then choose the Supplier.
4. From the Category column, select the expense account for the transaction.
5. Enter the description and amount of the expense and select the Billable checkbox.
6. In the Customer column, select the customer you want to bill for this expense.
7. Fill in the necessary field. Then, hit Save and close.

See this article for more information: Enter billable expenses.

In addition, here's an article to help you manage your bills in QBO: Enter bills and record bill payments in QuickBooks Online.

You can also check this page to learn more on how to handle supplier balance in QBO: Manage outstanding balances for customers and suppliers in QuickBooks Online.

@nurimran, don't hesitate to get back to me if you need further assistance with this. Our door is open 24/7, and I'll be happy to help you out again. Stay safe.