The small Sihanoukville Airport (IATA: KOS | ICAO: VDSV) is located 17km to the east of town, on the edge of Ream National Park. Cambodia Angkor Air started flying to and from Siem Reap on December 14th 2011 with three flights a week. They also fly nonstop to and from Vietnam (Ho Chi Minh City) 3-4 times per week. There are red-eye flights from Beijing (PEK) as well now. AirAsia began flying to and from Kuala Lumpur, Malaysia in August 2017, 4 times a week. Hainan Airlines offers direct service to KOS from BaoAn Airport (SZX) in Shenzhen, China.
Sokha Helicopters offers a VIP charter service between a private helipad at Sokha Beach Resort and Phnom Penh and Siem Reap. Charter rates start at US$1495 per flight hour plus 10% VAT & 10% SPT, with discount packages for stays at Sokha Beach Resort. Helicopters are modern, French-built Eurocopter Ecureuils with luxury leather seating for 5 passengers. Licenced pilots are from Australia and Europe. Flight time from Phnom Penh is 55 minutes and Siem Reap 1 hour and 40 minutes.
Sihanoukville being a famous destination for holidays for both locals and foreigners, there are many buses and minivans going there everyday and the road is usually in good condition. You can check the different schedules, prices and book directly online via camboticket.com
From Phnom Penh:
Capitol Tours runs multiple large buses a day taking about 5-5.5 hours. The cost is 20,000 riel (5 USD) as of Jan, 2016. The buses depart from Capitol Tours near Orussey market and tickets can be purchased there also. The seats on this bus are quite small, not really comfortable from two large adults sitting next to each other. The bus makes stops along the way to pick up people and goods. It will drop you off in downtown Sihanoukville, about 3 kilometers from Ochheuteal/Serendipity beach, motos and tuk tuks will be waiting there quoting 3-4 dollars for the trip to Ochheuteal/Serendipity. If you get past these and grab a motorbike it should be around 4-5000 riel, a tuk tuk should be 2 dollars, some drivers will stick to their ridiculous prices, so if that happens just move on, don't try to negotiate with them. (as of Jan, 2016)
Phnom Penh Sorya Transport, GST Express, and Mekong Express operate hourly bus services from Phnom Penh, taking 5- hours and costing US$4.5-10, depending on the quality of the bus and the number of stops. The first bus leaves Phnom Penh at 7:00AM; the last bus leaves at 5:45PM. All buses arrive and depart from the bus station downtown near the Central Market. Tickets can be booked at the bus company offices, travel agencies, and many guesthouses. It is worth reserving a day in advance to be sure of a seat. National Route 4 between Phnom Penh and Sihanoukville is one of Cambodia's best roads, despite this travel can be at a snails pace due to traffic. (undated info)
Giant Ibis and Mey Hong offer 14-16 seat air-conditioned minibus transport that is aimed at tourists and never have more passengers than seats. Tickets cost $10, although it can cost a few dollars more if you need pickup on Otres or far from their offices. (undated info)
Rith Mony, Bun Thou, and Virak Buntham Express operate daily air-conditioned minibus service leaving at 8:30AM to/from the the border with Thailand at Koh Kong/Hat Lek, taking about 5 hours. "Local" and "Tourist" minibuses service this route; they are always jam-packed, and the trip can be uncomfortable. "Local" service price depends on how much space you want (a whole seat, half a seat, or a space on the roof); foreigners can expect to pay around US$6-8. They will also typically have pickup in front of the offices on the main strip in town at 7:30AM (at which point they simply drive over to the station to wait until the 8:30AM departure time.) You may be offered pickup at your hotel for a premium. Sometimes there are buses leaving at 2:00PM but the service may be available only with suitable demand and ticket prices may be higher (~US$15). (undated info)
In 2010, Virak Buntham Express started a bus service from Ha Tien in Vietnam to Sihanoukville via the new Xa Xia/Prek Chak border post. (From Ha Tien, there is a ferry to Phu Quoc island, another popular tourist destination). Unlike even recent travel guides report, this trip now has become easy and straightforward. Visa on arrival for USD 25/550.000 VND (but bring a photo!)
From Siem Reap:
There are various company night buses running from Siem Reap. Trip takes around 12-14 hours and costs 14-17$ for a sleeping 70% reclining seats/hotel bus with beds. But beware that road is really bumpy, seats may be not like shown on photo in the ticket office, and you may be forced to change bus from sleeping to sitting in Phnom Penh, so make sure you have the direct bus with no changes, assigned seat and seller phone number to get what you paid for when you buy a ticket.
The bus station is far from the city centre and you will need to take a tuktuk or a motorbike. Expect to pay anywhere between $ 1 and 4, depending on how many people there are in the tuktuk.
Cambodia's rail network has recently been restored and provides a cheap and comfortable way to travel. Services between Phnom Penh and Sihanoukville restarted in April 2016. Tickets cost $7 each and the journey is more relaxing than the bus trip. The service is operated by Royal Railways.
A chartered taxi from Phnom Penh's Central Market can make the trip in less than three hours and will cost anywhere from US$25-40 per car, depending on the petrol price of the day and how beat up the vehicle is. You can reduce the price by sharing seats, but be warned that Khmers will squeeze as many as eight people into the car, including two in the drivers seat, so most people will need to buy two seats for comfort. As of May 2013, you can get a private taxi to Sihanoukville for $50 - $55. Many of these have an LPG tank in the boot, so one large case is the limit. If two people travel a spare seat can cater for a second large case. Most of the cars are relatively recent Toyota Camrys and comfortable enough. Order from Travel Agents around the city.
Boats used to run daily from Koh Kong/Hat Lek (the border crossing with Thailand), taking around 4 hours and costing US$20/700 baht. However, the service was suspended in 2008 and it's unclear if it will resume, since travel by road is now cheaper, safer and just as fast. Several sources quote "persistent rumours" about a future ferry service with the nearby Vietnamese Phu Quoc island, but nothing happened yet.
Boats to the islands offshore - Koh Rong and Koh Rong Samloem, run every day several times per day. You can take a slow boat, speed ferry or make a day out and go on a tour boat. Speed Ferries are the best option if you just want to get out to the island as quickly as possible, as the company runs modern purpose built catamarans with lots of under cover seating. Alternatively, get on a day trip tour boat and you should be able to use your ticket to get off at the island and stay overnight, and return another day.
Modest numbers of cruise lines offer one-day port visits, usually along with more stops in Vietnam enroute to or from Bangkok, e.g., Oceania.