There are no direct bus services between Obidos & Porto.
The current CP timetable show two daily connections by rail changing at Caldas da Rainha and Coimbra-B.
Departures as follows:
Dept Obidos: 11h03
Arr Porto Campanha: 14h39
Arr Porto Campanha: 20h35
More info is available from the Portuguese Railways website at cp.pt
The first two trains to Coimbra are regional trains and normally there are no reservations.
The Intercidades between Coimbra-B and Porto Campanha has to be booked.
This would mean getting a bus to Caldas, then to Coimbra, then Porto or back to Lisbon then onto Porto
Two different companies are necessary with the first leg by local bus.
Personally, I would recommend the train option.