The whole concept of symbols is Western. Like since Christians have the Cross sign, they went looking for signs in other religions that maybe equivalent of the cross sign. Signs are not inherent to many religions
The Om symbol is just one of the symbols, in fact it is not even such a widely used symbol in Hinduism. The swastika is much more widely used symbol in Hinduism. But since the western Nazis has spoiled the peaceful meaning attached to the Swastika, our Western friends have decided that Om is the proper symbol for Hinduism. So be it.