Travelling from Porto to Obidos is not recommended.
By train it takes 5 hours and 4 to 5 changes or trains.
There are no direct buses that I am aware off that run direct from Porto to Obidos.
I would strongly recommend therefore that you go direct from Porto to Lisbon and then make a day/afternoon excursions from Lisbon to Obidos.
I have uploaded an extract from the current timetable here:
http://www.scribd.com/doc/18635023/Lisboa-Obidos-Bus
and return:
http://www.scribd.com/doc/18635385/Obidos-Lisboa-Bus-Timetable
Note that Diariamente means "Daily". The rest are weekend and Bus Schedules.
Buses are operated by http://www.rodotejo.pt and leave from Campo Grande Bus/Metro Station.
