I would say it's more that while Worf has combat experience, it's more the kind of work that Security typically handles in Trek episodes. O'Brien actually served during a war and saw combat on a much larger scale.
Agreed. Worf holds the Klingon warrior tradition in high esteem, but in the sense of practicing a religion. He participates in ritual (various competitions, etc) and through training keeps himself in shape in accordance with the way of the warrior. That's not the same as actually having lived the warrior's life the way Klingons like Kor or Martok have. It makes him proficient with weapons and very good in a fight, but "warrior" is still an ideal for him; an aspect of identity, not a job description.
Worf's a security man who happens to live by a tradition that promotes physical prowess and combat. O'Brien, meanwhile, was an actual soldier and saw action in disputed territories, on the ground. He may be about 1/10th as martial in terms of his outlooks and interests, and he obviously couldn't compare to Worf in terms of weapons proficiency or physical contests, but in terms of the realities of combat outside of ritualized tradition (well, to the extent that warfare isn't always
ritualized tradition, but that's another matter...), I'd bet on O'Brien. Worf is often detatched from realities of non-"honourable" combat, perhaps especially emotional realities. He doesn't see combat and death primarily in terms of the people affected, he sees them as an abstract idealized frame of reference for his personal belief system.
On a related note, in terms of experience, I'd think O'Brien was more useful on that front than Worf any day, precisely because O'Brien cares nothing for combat but has still engaged in it. Who would you rather trust in, the person whose perceptions are welded to a preconceived and often idealized idea of how combat works, along with a long list of culturally-approved descriptors designed to warp the experience into butressing a ideological framework defining his people, or the guy who had no interest in combat beforehand (or afterward), and simply found himself engaging in it at one point in his career, doing the best he could, and who can share his personal experience without anything at stake for him beyond the act of advising?