To import Outlook OST to Office 365 using the Regain OST to Office 365 Migration Tool, follow these steps:
Download and install the Regain OST to Office 365 Migration Tool on your computer.
Launch the tool and select the option to "Add OST File" to load the Outlook OST file you want to migrate.
Provide your Office 365 credentials to establish a connection.
Choose the Office 365 mailbox where you want to import the data.
Start the migration process, and the tool will transfer the content from the OST file to your Office 365 account.
Know More:
https://www.regainsoftware.com/ost-to-office-365-migration.html