Altrimenti can be an adverb or a conjunction. As an adverbe it can' t be substituted by oppure, but used as a conjunction, that is possible.
adverbe
Altrimenti non lo faccio!
Otherwise I didn' t do it.
conjunction
Me lo dai adesso, oppure / altrimenti vado via.
You give it to me now, if not I go away.
It is obvious that in the first sentence altrimenti is an adverb. If there is no subordinate clause it is hard to imagine that altrimenti connects the main sentence to a subordinate clause. In this case it can refer only to the verb. It is quite obvious as well that altrimenti in this case is a pronominal adverb (for more detailed discussion see prepositions) because it refers to an idea, an ensemble of words and everybody taking part at the conversation must know what it refers to.
example
Altrimenti, oltre a non avere senso, si blocca la comunicazione prima che questa inizi.
Besides that it doesn't make any sense, the comunication is blocked before it has even started.
