Hi there, @simon22.
You can use the Estimate feature in your QuickBooks Online (QBO) account in creating a work order. While QBO doesn't refer the form as a work order, you'll find estimate as same.
You can use an estimate when you want to give your customers a quote or proposal for the work you plan to do. Let me guide you through the steps in creating an estimate.
In your QBO account:
- Click the (+) Plus icon.
- Select Estimate.
- Choose a customer.
- Enter the needed information.
- Once done, click Save.
Here's an article you can read for more details: Set Up and use Estimates.
Then, once your customer accepts the estimate, you can easily convert the estimate to an invoice. Let me guide you how.
- Go to the Sales menu.
- Select the All Sales tab.
- Find and open the estimate.
- Select Create invoice.
- Fill the needed details and click Save and close.

I've got you this article for more details: How to Convert an Estimate into an Invoice.
Also, I'm adding this great article for reference in case you want to create more than one invoice from a single estimate: Create Partial Invoices from an Estimate.
As always, feel free to reach out to us if you need further assistant. I'm always here to help.