On song
Definition 1 of 2
Pro Tip 1/3Creative Peak
Used when someone is doing their creative work exceptionally well. 
The writer was on song, publishing several bestsellers in a row.
Pro Tip 2/3Performance Context
Most often used in contexts where performance is visible, like sports or music. 
With three goals today, the striker is really on song.
Pro Tip 3/3Positive Vibes
Implies a sense of flow and positivity in the way things are going. 
When our team is on song, the work atmosphere is amazing.