There is no direct train service from Lisbon Airport to Obidos and Obidos station is situated outside of the town.
Public Transport Services outside Greater Lisbon start to shut down on weekends around 20:00
The last bus from Lisbon to Obidos on Saturdayleave Lisbon Campo Grande at 16:45.
As regards trains there is only one train that will serve as follows:
Lisboa - Gare Oriente Station (Ten Minutes by Taxi from Airport)
Dept: 19h57
Obidos
Arrive: 22h48
One changes at Sete Rios and Mira Sintra-melecas stations en route.
Be aware that there will probably be no taxis at the station when you arrive unless you arrange one beforehand.
I recommend you see the following article on my blog which also deals with this matter.
http://barlaventostravelblog.blogspot.com/search?q=praia+del+rey
or
http://barlaventostravelblog.blogspot.com/2008/06/lisbon-airport-to-praia-del-rey-resort.html
