Postiga sidefooted home in the 53rd minute from a Nani cross to put Portugal ahead of Denmark, who won 2-0 in Iceland, and Norway with all three teams on 10 points from five games.
"It was a good game, though not a great one. Our win cannot be questioned," Portugal coach Paulo Bento told reporters.
Norway had two shots in the first five minutes but Portugal soon took control with Raul Meireles sending a drive wide after seven minutes and Cristiano Ronaldo forcing Rune Jarstein to tip away a trademark dipping free-kick moments later.
The Scandinavians were struggling but managed to give Portugal another scare after 16 minutes when Bjorn Riise led a counter-attack and fed Erik Huseklepp, who fired a low shot that Eduardo had to stretch to palm away.
"From the beginning, we wanted to control them and we did this even though we could not always check our opponents' counter attacks," Bento said.
With the lively Ronaldo combining with Fabio Coentrao on the left, Portugal had plenty of possession to show off their trickery but the final pass often failed to reach striker Postiga, who struggled to find room in the box.
DIRECT APPROACH
Norway were quick to break with a direct approach whenever they found some breathing space and midfielder Morten Pedersen missed a golden chance to give them the lead by blasting over the bar after a poor clearance by Meireles in the 24th minute.
Ronaldo and Coentrao combined five minutes before halft-ime to create a smooth move with the busy left-back crossing for his captain to leap above the tall Norwegian defence but the forward's header went straight to the keeper.
Postiga finally made space on the edge of the box and got his shot in, forcing Jarstein to touch the ball away for a corner three minutes after the break, and a Nani drive was just wide moments later as Portugal tried to break the deadlock.
The pair ensured Portugal did just that soon afterwards when the winger weaved in a cross from the right for the pacy Postiga who stretched to sidefoot the ball into the net.
"It's a move we frequently practice in training, a low cross for a shot from the front post. It allowed us to achieve our objective, which was to go top," Postiga told reporters.
Portugal could not maintain their momentum after scoring and the game petered out in the latter stages with the cautious Norwegians failing to produce the threat of an equaliser.
The Portuguese had a disappointing start to the qualifying campaign but have recovered under Bento, who took over after Carlos Queiroz was sacked in September.
The side have won the three qualifiers they have played under Bento, and have played well in friendlies, especially the 4-0 thumping of world champions Spain in November.
"We have built an identity for this team, and that comes from stability," the coach said.
