System Center Advisor “OMS” offers multiple cmdlets , the full list of cmdlets could be found here 
Last week I was working with one my customers to register his SCOM Management Group to OMS, Due to the security restrictions we couldn’t register the Management Group via the Wizard. our last option was to use PowerShell , the cmdlet to register a Management Group is Register-SCAdvisor. The cmdlet requires a certificate in a PFX. don’t try to generate a certificate from your Azure portal , it is not going to work. this is how to get the PFX
login to your OMS Subscription http://mms.microsoft.com once you see the main dashboard in the URL replace what after the Microsoft.com/ in the URL bar with the following DownloadCertificate.ashx
so the URL will be
yourworkspacename.portal.microsoft.com/DownloadCertificate.ashx
In my case my workspace name is ContosoIG the URL will be
ContosoIG.portal.microsoft.com/DownloadCertificate.ashx
and hit enter
Before :-
After:- 
Save the cert in your SCOM management Server (i.e. C:\temp) 
In SCOM PowerShell Console Type the following
Register-SCAdvisor –CertificatePath "C:\temp\RegistrationCert.pfx"
The cmdlets returns True if it succeeded
Tags: Advisor, OMS, Register Offline, Register-SCAdvisor, SCOM