Generation O
n. The generation of people approximately 18 to 35 years old who voted for or supported Barack Obama in the 2008 United States presidential election.
President-elect Barack Obama has been very judicious thus far about mobilizing the vast coterie of supporters from his campaign. Generation "O," for Obama, is ready to do its part in putting the country on a more hopeful path.

The question is what Obama should ask of his supporters. Washington already expects 4 million people on the Mall for Obama's inauguration, but what will he say to activate his supporters?

As a dreary Thanksgiving comes and goes, one answer is to mobilize Generation O to help the nation's struggling nonprofit sector. Unlike the automobile industry, whose representatives were ferried to Washington on private jets, the nonprofit industry has yet to show up at all.
—Paul C. Light, “Obama Must Mobilize Supporters to Help Nonprofits,” The Washington Post, November 28, 2008
Generation O is that college kid at the White House gate early Wednesday morning, lifting his shirt to reveal ''Obama'' painted in red on his chest.

Or that stylized Obama T-shirt that makes irony look old, the ''Obama Girl'' on YouTube, or the thousands of notes on Barack Obama's Facebook page: ''U are the best!!!'' ''yeah, buddy.''

And, of course, Generation O is the president-elect himself. …

These young voters and those slightly older, who together may forever be known as Generation O, were the ground troops of the campaign.
—Damien Cave, “Generation O Gets Its Hopes Up,” The New York Times, November 09, 2008
2006 (earliest)
Generation O?
Salon's Shapiro writes, "the morning after Comet Obama slashed across" the NH sky 12/10, Dems "were still trying to sort out exactly what had happened" and "long-memoried political reporters struggled to find a parallel."
—“Oh, Cruel, Cruel Hero, How You Mock Us,” The Hotline, December 12, 2006
Many thanks to Laurie Mullikin for passing along this phrase.