Not really
They say you do but I haven't ever been asked ( I am not sure the avaerage tourist is asked anything on arrival, frankly). If you are asked I suppose you just say you have $$ or a creidt card to buy a ticket when you are sure of the date.
One caveat: if the requirement for a return ticket is known by the airline then even if they sell you a ticket, one-way, they may not let you fly one-way unless you show them you have an onward ticket, which may not be back to where you left, on them , but another airline, say Aeropostal SDQ- Caracas, etc. Any refusals by immigration for bad documents are the responsibility of the airline.
You might find it cheaper to buy a round trip ticket anyway. One way is often quite a lot.