When you create a new account in QuickBooks Online (QBO), you need to pick a day to start tracking your transactions. Your opening balance should be your real-life bank's total for whatever day you choose. Then, it'll summarise all your past transactions up to the opening balance date.
If your bank account is connected to the online banking feature in QBO, the system will automatically download the last 90 days (or more) of your transactions and uses them to enter your opening balance.
If your bank account isn't connected to online banking, you'll have to manually create a new one and enter your opening balance through your Chart of Accounts. Let me guide you how.
Go to the Gear icon.
Select Chart of Accounts.
Enter all the necessary details (i.e. account name, type, detail, and default tax code).
Go to the Balance and as of date sections to enter when you want to start tracking your finances.
When you're done, select Save and Close.
I've attached a screenshot below that shows the last three steps.
Additionally, I'd recommend reconciling your accounts every month. This is to effectively monitor the growth of your business along with your income and expenses. You can refer to this article for the detailed steps: Reconcile an account in QuickBooks Online. It also contains instructions about editing completed reconciliations.
Please know that you're always welcome to comment below if you have other concerns or follow-up inquiries about the banking feature in QBO. I'm just around to help. Take care always.