A few days after writing these tips and I have already had e-mails asking if you can cross the Cambodian-Vietnamese border in this area.
The honest answer is I don't know, but my guess is that it would not normally be possible.
There is a good track to the border near old Dak Dam village. I haven't been on the road the last kilometre or so, but from a helicopter it looked like it was all functioning. However, as I suspect that it is impossible to check visas online, it is almost certainly not permitted.
There are two border crossings further west, one very close to Snuol and one near Suong on the route back to Phnom Penh. Both would apear to be much more substantial crossing points but again I have not tried either. I understand that the Snuol to Memot area still - thirty years on - is an extremely sensitive military area. (If you Google "Snoul" [sic] you will get some idea why.
There is a bus service to Phnom Penh, via Snuol and Kampong Cham every day (several times) and the guest houses will make arrangements for you. Tickets can also be bought at the bus office on the main street (just by the curve of the hill). I don’t know where the pick-up/drop-off place is in Phnom Penh (we came here by car) but it is a tough journey between Snuol and Sen Monorem (around 4 hours for this stretch on poor condition graded roads). I guess that you can get off the bus at Snuk to change for buses to Siem Reap if you wish.
Many people also take the regular pick-up taxis which have only 5 passenger seats and then a whole lot of people hanging on in the back. This makes the journey much faster than the bus. One couple suggested that for the return to Phnom Penh they would get a pick up to Snuol then a bus for the rest of the way on the sealed road – seemed a good plan.
One couple were planning to stay overnight in Kaaey Siema to visit the Biodiversity Conservation Area and the Snuol Wildlife Reserve, but we never found out if they found anywhere to stay in that area. There really is very little between Ou Reang and Snuol.
The airport at Sen Monorem is not - as stated in the guidebooks - out of operation. There are no commercial air services, but there are both government flights (very occasionally) and flights operated by the MAF NGO using small planes. If you have contacts with MAF or other NGOs, you could try to see if you could buy a spare seat on a flight. A helicopter can be chartered for the flight from Phnom Penh, but at upwards of US$1200 per hour (it's about two hours by helicopter from Phnom Penh) this is only for those with a lot of money. The helipad is alongside the runway, close to the Sen Monorem roundabout. Note that depending on the helicopter used, it may also be necessary to preposition fuel for the helicopter here as well.
If, by any chance, you have your own aircraft, note that the runway is not secured for landing and two roads cross the runway. Local pilots use only the southerly-most 600m of the runway.
A few vehicles are available with driver in town but these are generally saloon cars. We paid $50 for one day, which is expensive but there is a heavy demand in the dry season. Locals get around by moped, but it is worth remembering that in the event of an accident, you are a day's travel away from even the most basic medical care.