There are more features to administer in Office 365. SharePoint Online is just one of them.
Your understanding of sites, web, metadata, content types, search, taxonomy will still be needed, if not more. In addition, you need to understand modern sites, hub sites, Office 365 Groups, Teams, Stream integration, Forms, Yammer/Teams/SharePoint's outer loop and inner loop.
Your skills with PowerShell, with PnP-PowerShell or SPO-cmdlets, will need to increase to include Azure and Exchange. You will run them in Azure Automation or Azure Functions, and build massive automation steps to manage an ever increasing set of triggers and webhooks. Some of these are building blocks only you as an admin can do, and you need to build these building blocks for your business analysts that are rolling out PowerApps, Flow or developers building custom SPFx webparts.
Your job as a SharePoint administrator is only finished if you consider there's nothing left to do. There is actually so much more to be done.