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I have not done it flying Singapore Airlines but used a one way ticket flying Korean Air (Delta ticket) without any problems.  I do have an ER visa (resident) for Cambodia, however, and you almost certainly have to show a visa at check-in.  And then, I am not sure whether a tourist visa would be sufficient to allow you on the flight.  If that's what you have, I'd not risk it and get a refundable or changeable roundtrip. 

Good luck!

I come to Cambodia every year on a one-way ticket and get the visa on arrival at the airport.  Simple and quick.  Singapore doesn't care what kind of ticket you have, just so you can show you are leaving.  If you transfer flights in Singapore that's all you're doing, transferring.  They don't care.

Be more concerned about your living quarters having a rooster outside your window.

@12HourChangeI would be taking Singapore Airlines from San Francisco to Siem Reap with a few hours  lay over in Singapore only.However, I originate my flying and check my luggage in San Diego.I just don't want any hassel upon check in San Diego with Sky West Airlines as Alaskaskywest.

I don't know about the connecting flights you have from San Diego but I have flown many times from SFO on Korea Airlines on oneway tickets. These flights go from SFO to Inchon to Phnom Penh. I have never been asked for a return flight proof. If I did I would just buy a best onward flight "ticket" for $12. You can find these vendors online. Just pick one up and you get an itinerary that's real out of Phnom Penh when you want. Just set it for 30 days or so after you arrive.  I think I was asked one time here in Asia on Air Asia for a onward flight proof. I used the service I mentioned.

Thanks Mike, I hope to be there no later than by the end of October.

@HamSR I have never been asked to show any visa flying into Cambodia on Korea Airlines or any other airline. I don't think getting a tourist visa online will allow the issuance of a retirement extension of stay. It is far better to get a visa on arrival and get the ordinary visa that is extendable to the retirement EOS or you risk having to leave Cambodia and return with a new visa.

@arthow1847

I come to Cambodia with a one-way ticket every year.  There is no problem at all. I get visa on arrival at the airport, easy and quick. Singapore doesn't care as long as you got the ticket going out.