For quite some time it was clear that the OMS Portal will move completely to Azure and that is good news. We have seen services like Update Management, Azure Security Center (Security & Audit solution is part of it) releasing new functionalities only in Azure Portal. In fact some services that have been part of OMS (OMS is a suite not a product or service) have always been in Azure Portal. Such services are Azure Backup, Site Recovery, Application Insights, etc. Microsoft has documented OMS Portal deprecation but I would like to add some things to the ones documented there:
Cloud Administrator in Azure World 