I do both OC and CC, depending on the situation, locale, etc. Lots more philosophy of OC at Opencarry.org forum a.k.a. OCDO. As in most debates, for each point one side makes, the other has a counterpoint. You have to study both sides and form your own opinion. Some examples:
POINT: CC gives the carrier the "tactical advantage" of surprise
COUNTERPOINT: Depends on the situation. Surprise is really only an advantage in an offensive scenario.
POINT: OC has a deterrent value. BG sees you're armed, and moves on in search of an easier target. (There is no documented evidence of this, because it's impossible to prove a negative, but there are many anecdotes of "suspicious" people suddenly changing their behavior when they become aware of an OCer.)
COUNTERPOINT: OC could make you a target. (Note, there are zero documented incidents wherein an OCer was "taken out first"; most criminals don't have that level of SA or courage. There are a very, very few anecdotes where a non-situationally-aware OCer was robbed of his gun and other valuables.)
POINT: Many people react negatively at the sight of a gun. OC will mobilize anti-gun activism.
COUNTERPOINT: "A right unexercised is a right lost." Until people see law-abiding citizens carrying guns on a regular basis, it will be alien and discomfiting.
IMHO, responsible OC places a higher burden on the carrier than CC. It requires greater situational awareness to avoid a potential "gun grab." A retention holster is prudent. Solid knowledge of the law is required, since sooner or later you will be engaged by an ignorant citizen or ignorant LEO. Like it or not, an OCer is representing all gun carriers in many people's eyes, so a calm, professional demeanor and exemplary manners are strongly recommended (Someone who is CCing and being a jerk is just a jerk. An OCer being a jerk is "that @&$*&% with a gun!").
And yes, there are jerks in every crowd. I don't doubt that there are OCers out there who do it to "show off," "act big," or "bait the police." That doesn't mean that every OCer is that way. There are jerks on every highway in America, but that doesn't mean we should all quit driving.
Others have already given some of the typical responses: comfort, speed of draw, education/advocacy, usually legal for 18-21 year-olds, and Because I Can. My favorite one is, it pisses off stupid people.
Lastly, it's been my experience that OCers don't care if you choose to OC or CC, as long as you carry, follow the law, and don't make the rest of us look bad. Some CCers, however, get very upset by OC and start to sound just like anti's. Just my $0.02.