A one-way flight to Seychelles can be a great start to an island trip, but you should arrange your departure plan before reaching the airport. Seychelles immigration lists a valid return or onward ticket among the conditions for a visitor's permit. That means your flight into Mahé is only half of the travel story. You also need to know how you intend to leave the islands, on what date, and where you will go next.
The useful question is not simply “Do I need an onward ticket?” It is “Which departure flight fits the trip I am actually planning?” Answer that first and the paperwork becomes much easier.