This depends on the airline. Some airlines are really strict about you having an onward ticket, some aren’t. Other than in the UK and Ireland, I haven’t ever been asked at immigration to show onward proof. Technically, every country has this requirement but it’s not really enforced. However, I know a few airlines that won’t let you on without onward proof, so sometimes it’s good to buy a refundable ticket or a cheap bus ticket out just to satisfy them.