Honestly, טוב often can mean 'okay' in Hebrew, but this is more of a colloquialism. The words are not fully interchangeable, and they do have different meanings, even though we can use them the same way in some situations to mean pretty much the same thing. In most cases, טוב is a bit more firm, and בסדר is less so. Here are a couple examples that I hope can help: הכל טוב? = הכל בסדר? (Is everything good? = Is everything okay?) טוב, אז מה אנחנו עושים? = Okay/right, so what are we doing?
The word בסדר literally means "in order". "Is everything in order?/Hakol b'seder?" We could substitute "ok, fine, all right," even "good" in this sentence. The word טוב just mean "good". "He is a good man/הוא איש טוב. Good morning/בוקר טוב." We cannot substitute בסדר for טוב in these sentences. So, just think of b'seder as meaning "in order" or "ok" or "all right" but not "good" and you won't go wrong.