Aum/Ohm/Om or however its supposed to spelled is often used to begin Hindu and Buddhist mantras. The Korean monk who drew this design told us that this is good for warding of ghosts. I have been told that this looks like Chinese style Siddham (a kind of Sanskrit). In other words, this kind of Sanskrit is designed to look more like a Chinese character. Koreans still occasionally use Chinese characters (Kore: Hanja) for various purposes so its not surprising that Korean calligraphy would be influenced by Chinese styles.