Which system do we enter our opening balances for debtors - QBO or Mechanic Desk. We have bought a workshop system which integrates with our accounting system?

I can help share a few things about this, @Mandy01.

Entering an opening balance for debtors depends on how the app works on QBO. 

If Mechanic Desk sync and import transactions in QuickBooks, you can enter it in the app. Just make sure to check if you can import the Chart of Accounts as well as the products and services. 

However, If it'll just sync transactions, you can enter the opening balance in QBO.

For other details about the workshop integration, you may check this link: QuickBooks App Details.

You can always drop by again if you have additional questions about entering an opening balance for debtors. I'd be happy to help.