My wife and I bought a one way ticket to Chiang Mai we're leaving in early December. We're applying for a single-entry visa at the Thai consulate here in the states and then plan on extending our stay for an additional month either by applying for an extension at immigration in Chaing Mai or looping through a neighboring country via land border. My question is, will a one way ticket raise a red flag? The last time we traveled to Thailand with a multi-entry visa (we did not over stay) we left NY with a one way ticket 
but we also had a prepaid ticket to Vietnam. Any advise or suggestion would be appreciated.

If you have a VISA, one way is OK.

I don't believe CM Immigration will ask for it (proof of onward travel within the visa time period) but your airline might [when I've had a one-way ticket before, my airline (Korean Air) asked me about it before but that was resolved by showing them my visa (well, actually an extension of stay)].  

Your airline may verify at check-in that you have a visa.  But as you do have a visa it will be just fine to board.

Thanks again everyone!

Unless you have a ticket right through to CM, you go through Immigration in Bangkok.

When applying for, leaving on, entering on a VISA you can be queried for an outbound flight-ticket at each step:

- When applying for the Visa at the Embassy/Consulate (but probably not needed when applying in US);

- When boarding the Thailand-bound plane (but showing the VISA will almost surely solve the issue);

- When entering Thailand (but because of the VISA you most probably will not be queried about it).

But might be a good idea to already have checked out the options to buy an on-line outbound-ticket on the spot in the unlikely case that you are being queried.  E.g. AirAsia on-line ticket HuaHin to Kualu Lumpur for under 30 US $

thanks again!

Don't even think about mentioning your planned extension when going through immigration.

I have been coming to cnx since 2008 with always a ONE way tix...only been asked once at LAX about a visa, and they let me go as I always say I leave after 30 which my passport reflects..

 

30 stamp at bkk--extend at cnx and head to laos for visa-extend--go home....always on a one way tix