Many countries have a theoretical requirement that you have an onward ticket, which they don't really enforce... BUT the airlines are often paranoid as hell about this.
If you arrive in a country and are refused entry because of paperwork ( lack of visa or onward ticket, for instance ) then in many countries the airline is responsible for you. They have to pay a fine, and provide you with transport to somewhere you are allowed to be.
So the airlines often check your documents when you check in for the flight. If you don't have an onward ticket, they may refuse to let you fly, even though they were happy to sell you a one-way ticket. I had to deal with this bullshit twice last year, in Asia, and again the year before. Airline staff are often poorly-trained about this, and would rather create a problem for you than take the chance of making a mistake.
I would consider buying a return ticket just as a security precaution, they're often not much more expensive than a one-way flight and also useful in an emergency. Alternatively, buy a separate full-price one way ticket home that is fully refundable. After your trip, you get your money back, if you come home early enough and don't mind paying the fees involved.