It's hard to break down what OM is not because it is so complex, but because it is so simple. Music journalists are absorbed by terminology and context. We see a band like OM featuring the bass player (and, in a sense, the heartbeat) of Sleep, Al Cisneros, and immediately want to suggest "stoner" connotations. We hear the chant like pieces featured in the duo's songs and want to call it "eastern" or "Byzantine." I personally like to refer to the band as being "pantheistic meditation-metal."
This is fine. We are writing through the lens of a very rational and precise culture and we want to have strong frames of reference when we communicate things to other people.