There is an awesome article explaining on migrating from SharePoint on-premise to online Office 365.
SharePoint to Office 365 Migration Supported Scenarios
This article should help you in understanding the steps involved. Reading the complete article would definitely help you but if you can read just one section then you will understand which is the best way to do the online migration of SharePoint.
"What are the Supported Upgrade Scenarios to move to SharePoint Online or Office 365?"
Do not miss this section in the article.
To retain the metadata, a third party tool is the only approach I know when migrating to Office 365.
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Edited by
Zakir Chougle
Thursday, May 21, 2015 11:35 AM