Would I be able to post a transaction in Quicks Book online even after setting a closing date?

Yes, you can definitely post a transaction after you set up a closing date, phillipa-launchp.

Some changes can be entered after your book is closed. There are legitimate reasons for changes to be made to your closed books. Your tax professional most likely will provide you with adjusting journal entries. 

When you set a closing date from settings, there are options on how you want to handle it. You can select either Allow changes after viewing a warning or Allow changes after viewing a warning and entering a password. It's how QuickBooks manage your accounting data before the closing date. It still allows you to modify your previous transactions.

A user from your accounting team possibly has access or permission to amend the journal entry or post a new transaction dated before the closing date. A primary admin and company admin can do it. Still, they'll see a warning message that this can affect your books. 

To create a new transaction or change the existing one, click Yes. If you're required to enter a password, you'll have to enter it. 

I've gathered some articles for your reference about the closed book and Year-end guide for QuickBooks Online:

• Edit your closed books.
• Year-end guide for QuickBooks Online.

I'm always happy to provide more information about QuickBooks Online. If you have more concerns, please don't hesitate to let me know. I'll be here ready to help you manage your closed book and newly entered transactions. Take care always.