Groups and sites listed in Outlook are different than what appears in OneDrive for Business (new/modern experience). In Outlook all the Office 365 groups are shown that the user is a member of, however, in OneDrive for Business modern view it will only show a few.
* If you switch to the classic OneDrive experience then all the Office 365 groups are shown.
To have the group sites appear in the OneDrive list, navigate to the mail icon and select a link under Groups to go to the group site.
Once there, select More and choose Site:
Then click on the Star “Not following” (highlighted below):
This will add the group to the sites that appear in the OneDrive modern view.
*This behavior is by design for the Modern experience in OneDrive for Business.