In the book, the line first comes about when Oceania makes peace with Eurasia and declares war on Eastasia midway through the book. The protagonist sees people happily ripping down anti-Eurasia signs and replacing them with anti-Eastasia signs. All the while, the speakers blare, "Oceania has always been at war with Eastasia." No one questions this, even though they surely know it isn't true. After all, you wouldn't want the Thought Police thinking you were remembering history incorrectly, would you?
This somewhat common line comes up occasionally in geek conversation,
usually in a sarcastic sense poking fun at coverups or changes in policy
that aren't admitted by the government or other authority figures.
"Everyone knows that Christopher Columbus discovered America
and was a great hero. He didn't kill any Native Americans, he was
nice to them! Oceania has always been at war with Eastasia."
"GTKY nodes are not acceptable, nor have they ever been.
Oceania has always been at war with Eastasia."
"Al Gore invented the Internet. Arpanet? What's
that? Oceania has always been at war with Eastasia." | 
