Can you manage multiple company's books from one user account in Quickbooks Online?

This is just the place to get the answers you're looking for, @userchristian.

Yes, you can have as many companies as you'd like under the same QuickBooks Online account. Each company you create requires an additional subscription, but you can access them with the same sign-in info. This lets you quickly switch between companies so you can manage everything more efficiently. 

To add a new company:

1. Go to the QuickBooks pricing page and select the subscription you want. 
2. You’ll see a page that asks you to create an Intuit account to buy QuickBooks. Scroll all the way to the bottom and look for the small text that says “Adding a company to an existing account.” Select Sign in next to this. 
   
3. If you're signed in to QuickBooks, you’ll be asked to confirm the account you want to sign in with. If the account displayed is the one you want to use for the new company file, select Continue.
4. When asked, enter the same ID and password you already use for QuickBooks. 
5. Fill out all the information about your new company and you’re good to go.

Now that you have multiple companies, you’ll be asked which company you want to open whenever you Sign in to QuickBooks Online . To switch between companies at any time, select the Gear icon and select Switch company.

To learn more about managing multiple companies, check out the following link. It has a bunch of useful information for you and your business: Managing your companies. 

Be sure to let me know if you have concerns while working in QBO. I'm here to lend a helping hand. Have a good one.