Is it possible to custom sharepoint online personal site (my site)

Will # _________ H As far as I know my site profiles for online have been functionally replaced with Delve Profiles and these can not be visually customized. The historical "my site" site is actually the host for users OneDrive (or at least the URLs tend to suggest this) so no visual custom here neither.

I haven't touched the my sites on the server version (onprem) in a while so I'm not sure of how it translates now in the latest versions. In the past I've wrote this article (https://sharepointallday.blogspot.com/2013/04/branding-sharepoint-2013-my-sites-with.html) for branding my sites using "feature stapling" when it was still based on the old school "site template" provisioning mechanics with masterpages. Now that all the hype is about the "Modern Experience" that no longer relies on ASP.NET Masterpages I'm pretty sure it doesn't apply now.

Integration of Exchange Online will require setting up the User Profile Service in your "onprem" SharePoint Farm in order to sync user's email (yes it's onprem technology, even though it's hosted in azure). If you don't features like mail notification will not work.

For OOS SharePoint has only very basic features for limited formats, if you need OOS you will have to build a dedicated server for this purpose and setup your SP Farm to use it (https://docs.microsoft.com/en-us/officeonlineserver/configure-office-online-server-for-sharepoint-server-2016/configure-office-online-server-for-sharepoint-server-2016).