O'Brien is in the IVAW group.
But Kerry did not invite O'Brien, who opposes both the Iraq and Afghanistan wars, even though Kerry invited O'Brien to stay at his Nantucket home in 2006 during a film festival featuring an antiwar documentary that O'Brien was in.
Last year, O'Brien organized his own Winter Soldier hearings featuring testimony from soldiers about how "extremely loose rules of engagement" and air strikes in Afghanistan kill civilians and alienate the population.
"I think we presented clear evidence that soldiers were being ordered to do terrible things," he said. "But there wasn't much of a response."
Members of the group testified before the Congressional Progressive Caucus, but have never been invited to an official hearing.
A spokesman for Kerry's office said he is "looking for perspectives from troops who have spent time on the ground, without regard to their opinions about the war overall."
In an interview, Kerry said it is important "to let democracy work, in terms of airing differences and options."
Kerry is calling one witness who will urge a dramatic policy shift: Andrew Bacevich, a Boston University professor and Vietnam veteran who lost a son in Iraq.
The reason why I say it was probably best to not invite the IVAW guys is that they are kind of undisciplined and emotional. I understand their anger, but that kind of testimony will not help in this day and age.