I did two trips with Nan Ladong, a guide with Sam Veasna: a morning of open-country birding south of Siem Reap, and a full-day trip to the dry forest and evergreen forest near Kbal Spean.
Both trips were great, and we saw just over 100sp in this short time. For the Siem Reap tour, transport was by Tuktuk since it was close by, and for the Kbal Spean tour we took a 4WD car (no dirt roads though).
I got quite a few targets, including Collared Falconet, 4 Prinia sp., Puff-throated and Chestnut-capped Babblers, Crested Serpent-Eagle, Black-headed Woodpecker and many more. Ladong knew every vocalization and specific spots for each species. Included in the Kbal Spean visit was a tour of the excellent ACCB rehabilitation center, an ex-situ conservation site for many species of endangered birds, mammals and reptiles.
I will definitely be calling Sam Veasna again when I return to Cambodia, as they offer quite a few more extensive trips for all the Cambodian specialties.