I currently have a 3 month non-immigrant O Visa which expires on February 1.

Correct me if am wrong, but if I leave the country, I can simply re-enter without a Visa for 30 days as I am a Canadian Citizen.

I plan to drive to Cambodia, spend the day at the Casino, and then return at night. I don't think that I need to visit any immigration office or the like?

Originally, I had planned to get a one year marriage visa, (400,000 Baht and all), but now I have a job possibility where they will send me overseas to apply for hte business Visa, and then give me a work permit. But first I have to briefly extend my stay here. The one year visa, was a lot of stress especially if I still have to get a business Visa.

any thoughts?

You do not have to get another visa if you get the wife support extension.

If you are married then the 400k support is the proper visa for you if you do not want to be tied to a job that could end at any time (assuming the marriage has better chance of course).

You can work on an O visa just as a B visa and you would not lose the O extension if your job were to end. A work permit can be obtained for either type of visa.

If you prefer the B visa yes you should be able to receive 30 day stay at the border immigration post when you return from Cambodia. But you now will have to leave Thailand and obtain a new non immigrant visa before any work.