SharePoint Online has some great simple project management features to be aware of:
1) It can be utilized as a central repository for project documents
2) It can be utilized to store and link to project plans (via Planner or via those created via a list)
3) It can be utilized to store calendar based information
4) It can be utilized as an issue log using out of the box list templates
5) It can be utilized for timesheet management via stored documents or created lists
6) It can be utilized to store project tasks via links to Planner, stored documents or via created lists
7) It can be utilized to store documents and make them manageable via a proper meta-tag-based strategy by type of document