The SihanoukVille Old Night Market is located at one of the 7 corners of the Golden Lions Circle by Serendipity / Ocheteaul Beaches.
It's open in the afternoon and evening, and has small streetside restaurants serving Khmer and Western Food. Also, a good collection of deep fried bugs, beetles, crickets, and snakes.
The food is generally good and cheap, but there's not a lot to do there. Worth a stop, if you're passing by or hungry. Very popular for local Khmer food and BBQ.