Eastern Cambodia currently has four international border crossings: one with Laos, and three with Vietnam. All four are open daily from 7am to 5pm. Entering Laos and Cambodia, visas are issued on arrival (roughly $30–40 for a Lao visa, depending on your nationality; $20 for a Cambodian visa). Heading into Vietnam, you’ll need to have acquired a visa in advance, since none are issued at the border.
Trapeang Kriel–Nong Nok Khiene Popular crossing 57km north of Stung Treng.
O Yadaw–Le Tanh The most useful of the three border crossings into Vietnam, 70km east of Banlung along a good road. Guesthouses in Banlung sell through bus tickets from Banlung to the town of Pleiku, in the central highlands of Vietnam (around a 6–7hr journey from Banlung).
Trapeang Phlong–Xa Mat Little-used (and difficult to reach) crossing around 70km east of Kompong Cham.
Trapeang Sre–Loc Ninh Around 20km southeast of Snuol. This obscure crossing isn’t of much practical use given the lack of public transport on both sides of the border.