Generally the U.S. tax system likes to tax options based on whatever makes the most money for the U.S. tax system. :-)
Until I know more about the specific stock option program you have, you should assume that the U.S. will want to tax you if (1) the option grant occurred while you were employed in the USA; or (2) the options vested while you were employed in the USA; or (3) both; or (4) you saw a picture of a U.S. tourist landmark on television when you were 8 years old. I'm just kidding about #4.
The date of exercise of options -- and where you are living when you exercise the options -- is usually unimportant.
Meta point. The people who REALLY care about taxation of options are the people who run the company you work for. Your employer is going to bend over backwards to sacrifice you to the Almighty Tax Gods if it is going to protect the company from potential tax liability, even if it costs you money. Just sayin'. This is what I've seen.
Until I know more about the specific stock option program you have, you should assume that the U.S. will want to tax you if (1) the option grant occurred while you were employed in the USA; or (2) the options vested while you were employed in the USA; or (3) both; or (4) you saw a picture of a U.S. tourist landmark on television when you were 8 years old. I'm just kidding about #4.
The date of exercise of options -- and where you are living when you exercise the options -- is usually unimportant.
Meta point. The people who REALLY care about taxation of options are the people who run the company you work for. Your employer is going to bend over backwards to sacrifice you to the Almighty Tax Gods if it is going to protect the company from potential tax liability, even if it costs you money. Just sayin'. This is what I've seen.