It is recommended to apply for an e-visa for Cambodia in advance. The visa is valid for 3 months, with single entry, and a maximum stay of up to 30 days. It can be processed as quickly as within 2 hours. You can apply through the website:https://m.visageniusglobal.com/product/10030?identity_id=0  

Alternatively, you can also queue at the VOA office in customs and obtain a visa on arrival at Cambodia's international airports or border checkpoints, which usually needs to wait for approximatly 1hour-2hours.