Do I need to show a flight or will a bus ticket be sufficient for proof of onward travel with a visa on arrival in Thailand?

Rebecca - do you mean 'visa on arrival' or visa exemption? The difference is the passport you hold. Your profile suggest your from the UK? -- If that is the case then you don't get visa on arrival you'll get visa exemption. I think the larger issue is not so much Thailand's rules about entry into the kingdom.. They'll let any half-decent money spending tourist in (few exceptions), but will the airlines let you board? In the US some airlines will not let you on without A) a visa or B) all the requirements. In this case - its a matter of what the airline will accept. Call them. You may find your question better answered on this forum if asked as: "I am a UK passport holder. Planning to visit Th for ... days. I will be there for [fun/tourism,work,education, etc ]"

For the Visa on Arrival you must have a confirmed return ticket within 15 days. This Visa is only for 21 countries, the details you can read here:

bestonwardticket.com

I use bus tickets BKK to Phnom Pehn the site is 12go (google bus tickets BKK to PP ). They can be purchased online about $20 USD.