If I understood well your question, the case whose ending is -OM is instrumental, the 6th case that denote with whom you do something or by means of what you do something. The preposition is usually SA, but if you want to say by means of, that the instrumental is used without prepositions. Razgovaram sa prijateljicom. Dolazim autobusom.

OM is going to use in Instrumental and locative case just like Tina and Ivana has just explained to you.. Instrumental represents ALWAYS the company. For example. Ja sam sa SuzanOM, MarkOM, ... - I am with Suzana, Marko . The locative case is used as an object of the following prepositions: • Prepositions: u ‘in’ and na ‘on/at’, po ‘by, through, on’ indicating the position or location of the object • Preposition: o ‘about, around’, with verbs of ‘talking’, and to designate a location. • Preposition: prema ‘toward’, pri ‘at, over, by’, indicating the place or location of the object or the time of the action or state.

It's not only instrumental. Also it can be lokativ (as far as your example talks about "na hrvatskom"). For example: I'm talking in Croatian - Pričam (or razgovaram) na hrvatskom. Ivana wrote that instrumental is 6th case but in Croatian is 7th while lokativ is 6th. I think that in Bosnian language instrumental is 6th but I'm not sure. Lokativ has the same questios like dativ but lokativ has some specific prepositions which help to recognize. One of them is "na". Final, it's not easy to answer at your question since are endings depend on gender. For example, feminime gender doesn't have -om in lokativ.