To my knowledge, that particular character always stands alone to represent "om" specifically as a sacred syllable used to start mantras, etc. Of course, that particular combination of sounds occurs elsewhere in Sanskrit, and when that happens it is written differently. For example, "karomi," meaning "I do" has the same sound combination as the sacred syllable "om" in it, but I have never heard of anyone claiming that this is an actual incidence of the sacred syllable. Likewise, "karomi" would be written करोमि in Devanagari script, with the letters "om" being represented as " ोम" rather than "ॐ."
I probably made that more complicated than I needed to. The gist is this: "ॐ" does not occur in the middle of any word that I know of, but things that by coincidence sound just like it but are written differently do.