In Hindu, Buddhist and Jain traditions Om is the sacred syllable, the sound from which all things are said to arise. Seeing or hearing it in a dream is read as a call to stillness and a sense of being centred.
Does the traditional reading fit your dream, or did it mean something else to you?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.