opening balance overdraft bank account

It's a pleasure to have you here today, Marlin1. 

You can create a new liability account to record the balance of the overdraft. I'll guide you on how. 

1. Go to Accounting from the left menu.
2. Select Chart of Accounts.
3. Click New. 
4. From the Account Type ▼ dropdown menu, select Current Liabilities.
5. From the Detail Type ▼ dropdown menu, select Line of Credit.
6. (Optional) Change the account name.
7. Select Save and Close. 

For the detailed instructions, see Step 1 from the Set up the accounts for your line of credit section through this article: Set up and track a line of credit in QuickBooks Online. It also provides you with more information on how to track your LOC. 

On top of that, I've also included this reference for a compilation of articles you can use while working with us: Know where you stand financially.

If there's anything else that I can help you with, please let me know in the comments below. I'll be here to lend a hand. Take care!