Sounds like you will have more than 12 months to make a return flight. Are you 100% certain that over 12+ months period that there is no chance that you will return to origin (home) for extended weekend or something like that?

If such is the case then maybe it is best to buy two separate tickets. It is against the ticket rule, but for international it is possible to find cheapest round trip fare which is less than full fare one way. Purchase cheap round trip fare to reach your destination, throw away return portion. When you are ready to return home after 12+ months, buy another cheap round trip ticket to get home, throwaway second half of the ticket.

Above will be likely cheaper than true open return ticket which usually has to book into most expensive full fare. And as other has mentioned that even most expensive full fare is valid for only 12 months. Also, if you are going to buy most expensive full fare then there will be no difference buying one round trip or two one way. There is no point buying round trip open return. Just buy one way ticket to get to the destination. Buy another one way ticket when you plan to go home.