I have an Office 365/SharePoint Online app - not a SP Provider Hosted App registered with AppRegNew, but using AAD, OAUTH and the Graph endpoint to interact with SP data.
Now i'm looking for approaches on how we can connect that app to SharePoint on-prem. I know there's firewalls / connectivity to think about - but assuming that was all ok? Can I use the access token given by AAD to access SP On Prem in some way?
no idea, but really interested in your scenario. I found this video, and the title looks promising "Developing Hybrid apps for SharePoint: apps that work on-premises and in the cloud " https://www.youtube.com/watch?v=_U2Z5Ro9SNk