Hanoi is 5 hours ahead of Oslo
Best overlap: 8:00 AM to 1:00 PM in Oslo is 1:00 PM to 6:00 PM in Hanoi.
Your shared working hours run 8:00 AM to 1:00 PM in Oslo (1:00 PM to 6:00 PM in Hanoi). You've got a decent 5-hour window. Set a recurring time near 8:00 AM in Oslo (1:00 PM in Hanoi) and stick to it. Heads up: because Oslo changes its clocks and Hanoi doesn't, the difference moves by an hour twice a year. Confirm the overlap around the DST switch.