The YouTube API offers notifications to a HTTP callback on new/updated videos in a channel using Pubsubhubbub (https://developers.google.com/youtube/v3/guides/push_notifications).
Is there a way to authenticate as a user/channel (who authorized my app via OAuth2) when subscribing in order to also retrieve updates on their non-public videos?
In other words: Can i get some kind of push notification when a user (for whom i can provide OAuth credentials) uploads a new private video to their YouTube channel?
No, this isn't possible via the YouTube Pubsubhubbub, as it operates on public data and does not support authentication.
To know via API when new private videos are uploaded, you would need to poll search.list while authenticated as the channel in question.