on (prep): 19 USING EQUIPMENT using a machine or piece of equipment
He’s been on the computer all afternoon.
Is Rachel still on the phone?
work on somebody/something phrasal verb
1 to spend time working in order to produce or repair something
He has spent the last two years working on a book about childcare.
Every weekend you see him working on his car.
2 to try very hard to improve or achieve something
A trainer has been brought in to work on her fitness.
work on doing something
We need to work on ensuring that the children feel safe and confident.
So, "he is working on his computer" may mean
- He is using his computer as a mean to do his work.
- He is repairing his computer (changing the Ram)