There is only one border crossing from Laos to Cambodia, and it is a hard (corrupted) one. This is some information about the border crossing (including transport, visa on arrival and scams) completed with my experience crossing there in January 2015, and updated with the feedback from other travelers in 2016.
Border crossing Chiang Khong (Thailand) – Huay Xai (Laos)
The border crossing from Chiang Khong (Thailand) to Huay Xai (Laos) changed in December 2013. A lot of information in guides and websites is not up-to-date, so here there is some information on how to cross, based on my trip in Dec 2014 and updates in Jan 2016.