Is there a way to do this in SharePoint Online? Using Powershell for instance?
You can't use PowerShell with SharePoint Online.
You don't have access to the physical machine.
However, to set the navigation to share the parent's top link bar, you can use the Client Object Model if you want to go through the work of setting up the authentication mechanism.
NOTE! this is completely taken from @rjcup3 , so if this works please bump his answer instead :)
If you only want to do this once, and don't want to open Visual Studio you could try running what @rjcup3 said in your browsers console: