There are two different symbols here - the ok symbol, and the ones that you see in heavy metal concerts that people think is the devil's sing. Some more information on the mano curnuta hand gesture, from Wilkepdia.
Black Sabbath's singer Ronnie James Dio popularised the mano cornuta hand gesture, which has since become a symbol of heavy metal music in general. The corna (Italian for horns, also mano cornuta, horned hand and fare le corna, to make the horns) is a gesture with a vulgar meaning in Mediterranean countries and a variety of meanings and uses in other cultures. Its origins can be traced to Ancient Greece. It is realized by extending the index and little fingers while holding the middle and ring fingers down with the thumb.
The corna is not to be confused as the sign for "I love you" in American Sign Language, which is made by also extending the thumb, or the shaka sign used in Hawaii, which is made by extending just the thumb and little finger. While the "Hook 'em Horns" sign used by fans of University of Texas athletics is the same sign visually, it is used in different context and should not be confused with either.
It also has a variety of meanings in heavy metal and is known by a variety of terms: devil sign, devil horns, goat horns, metal horns, metal sign, throwing the goat, evil fingers, Pommesgabel (Pommes = fries, Gabel = fork), mano cornuto, peka evil, the horns, also "Rock Fist" or Rock on "Visel" in some U.S. rock music subcultures.
Gene Simmons claimed to have first introduced the sign to heavy metal, when he was shown extending his little finger on the cover of KISS' 1977 album Love Gun, although his claim is disputed. On the other hand Gene Simmons has, even before the Love Gun LP, showed this sign on stage or during photo sessions (for example Japan, April 1977). Gene was inspired by 60's Marvel comics, especially Dr Strange and also Spider-Man.
In heavy metal music, Ronnie James Dio also used this sign. His Italian grandmother used it to ward off the evil eye (malocchio). Dio began using the sign soon after joining (1979) the metal band Black Sabbath. The previous singer in the band, Ozzy Osbourne, was rather well known at using the "peace" sign at concerts, raising the index and middle finger in the form of a V. Dio, in an attempt to connect with the fans in a similar manner to Ozzy, wanted to use a similar hand gesture. However, not wanting to copy Osbourne, he chose to use the sign his grandmother always made. Both accounts are predated by the Chicago based psychedelic-Goth band Coven, led by singer Jinx Dawson who made the gesture on their 1969 album cover for "Witchcraft Destroys Minds and Reaps Souls". Incidentally, the band also wrote a song called "Black Sabbath," and one of the band members was named Oz Osborne, not to be confused with Ozzy Osbourne of Black Sabbath fame.
As for the ok sign, perhaps its time to get all bodybuilders to do the ok symbol instead of the thumbs up symbol!
