No, you cannot directly open or import a 3GB OLM file into Outlook on the Web (Office 365). To open or access an OLM file, you first need to convert the OLM file to PST format, then import it into your Office 365 Account, but this method is time-consuming and has a risk of data loss. So, the best solution to import OLM files to an Office 365 Account directly is to utilise a professional tool like
Softaken OLM to IMAP Migration Software. This smart application smoothly imports your 3GB or any large-sized OLM files to O365 or any IMAP Server Account without any restrictions. Also, these utilities are useful for all users, whether they are tech experts or not tech users