Do I need to apply for a tourist visa ahead of time for my visit to Koh Samui with a Romanian passport, or can I get a visa on arrival in Bangkok?

Just an unnecessary additional info.

IF there is any possibility for you to want to stay Thailand longer than 30 days (15 days after April), better to get Single Entry tourist visa. VOA is not eligible for tourism extension.

From an earlier POSTED reply.

The O/P clearly asked about a "Visa on Arrival" and that is something SOLD to people from 17 countries when they land here for 2,000 Thai baht. It used to be issued for 15 days but until Mar 31st is now 30 days.

These are the countries that BUY a Visa On Arrival to Thailand when they arrive here;

Bhutan, Bulgaria, China, Cyprus, Ethiopia, Fiji, Georgia, India, Kazakhstan, Malta, Mexico, Nauru, Paupa New Guinea, Romania, Taiwan, Uzbekistan & Vanuatu

That means if you come from one of those countries you DO buy a visa on arrival to Thailand and the price IS 2,000 Thai baht.

you need to show

...proof of 10,000 Thai baht or the equivalent in CASH,

...proof of onward travel in 30 days

...AND proof of lodging when you go to get the VOA issued at passport control

As a passport holder from Romania you are eligible for the VOA Visa on Arrival into Thailand. NO need to get a Thai tourist visa. On arrival into Thailand go to the VOA Visa on Arrival desk and apply.

There are NO EXTENSIONS allowed. You must leave Thailand within 30 days if you arrive on or before March 31....

FROM April 1 it's 15 days stamp as VOA Visa on Arrival.