We were bored with the movie, so we left.
I don't want to go, so I won't.
There are no more chairs available, so you'll have to stand.
It was still painful so I went to see a doctor.
There are commas in M-W Learner’s examples before so; in OALD isn't. Is it a different way to write in two countries, or either way do you, American and British, can write?