Book Bus Ticket From Phnom Penh to Siem Reap online?

I don't, because I never travel on a schedule. But, if YOU want to, try http://catmekongexpress.com

Haven't done it online but I've found that one company at least (Golden Bayon Express) charges more for online bookings. In fact on my recent trip to Battambang I paid $10 and their online listed price is $12. In the meantime my Cambodian colleague was charged $8. I know they also do Phnom Penh to Siem Reap but I've never used them for that trip.

Hi,

I didn't buy tickets from PP to SR but did reserve them online for my trip from PP to Kampot via the Phnom Penh Sorya Transport website (http://www.ppsoryatransport.com).

There's a form you need to fill in and you get an almost immediate response via e-mail.
No need to pay anything online: they will put your name on a reservation list at their bus station in PP.
Simply go to the bus stand 1hr before departure to pay your ticket.

Worked like a charm :)