I have a non-immigrant O visa (spouse visa) and have been working with a workpermit for an employer (school) for a long time. Now I'm thinking about joining a company in the USA (which my current employer already knows about) and breach the contract.

I've read about certain teachers that can not leave the country without a letter stating the date you left the school. Is this really necessary? Would officials stop you at the airport? And if yes, would it make any difference if you do a walk-in into one of the neigbouring countries and fly from there to the USA instead?

I heard stories about certain schools delaying to give a letter on purpose forcing the teacher to pay huge amounts on overstaying fees afterwards when he's getting back home and one person told me that was the school's way of forcing him to work for them longer.

Do you have a "visa" or an extension of stay ?

There is nothing your employer can do to prevent your leaving but it would be polite to provide notice of your departure.

There will be no "problem" at the airport.

I have a non-immigrant O visa (1 year spouse visa) that I've extended several times (border run, in out, get new stamp).

The workpermit has been obtained separately after entering Thailand before starting to work.

OK!

So you have a multiple entry non "O" visa based on marriage.

There is nothing which will prevent your leaving if that is what you wish.

Your visa allows you to leave the country at any time .

No worries at all based on what you posted. Out of professional courtesy give proper notice, pack up and Good Luck at your new job. Have a safe trip back.

Since you do not have an extension based upon working you will not have a problem leaving. At a couple of border crossing if you had an extension based upon working you would be asked for a termination letter.