Designed to be fast - The Outlook on the web user experience gets simpler and more powerful

@Counie  With Outlook on the web, similar to OWA when working with on prem Exchange servers, you access the mailbox directly on the server using your web browser. The .ost file is a local, synchronized copy of the mailbox contents that exists only when you use the Outlook desktop client and choose to cache the mailbox locally to allow offline access to the mailbox, i.e., access without an Internet connection. With Outlook on the web, no content is synchronized to any client, so there is no .ost file.

 

Thanks Kreera_House ,

Perhaps I didn't phrase my question appropriately . It wasn't intended to be a question about what an OST file was.

my query is , in relation to the article "Designed to be Fast" is that if there are known/documented performance issues related to our having OST file s stored locally for use with the Outlook Desktop product , on any hardware ( as per the support article) , above a specific size, is it possible that there are performance issues/recommendations when we go above certain mailbox sizes when using OWA (without OST files) in the cloud?

Or to be more direct:

Do we know what are the mailbox size to performance best practices in relation to OWA and O365? 

ps: my background is SharePoint btw and not Exchange so while I did know about OST files.. I'm not aware of all the guidelines that Exchange admins may have been aware of until now.. e.g. don't have more than X number of items in the inbox?