Here is the best KDETools OLM to Gmail Migration tool is an advanced utility in order to transfer multiple mailboxes from OLM to Gmail account directly including all emails, contacts, calendar, task, notes, attachments, journals, folders etc. This software maintains meta properties of the email such as (to, bcc, cc, subject, date, attachment and etc.) during the conversion process.https://www.kdetools.com/olm/gmail/