This author would say that in English there is only an open o as in words like cause, all, saw and the closed o doesn't exist. However that shouldn't be a big problem. For an unknwon reason people can imitate even sounds that they don't have in their mother tongue.
closed: avô [o] = (like Rose in german)
open: avó [ɔ] = bot
Try to hear the difference.
If you didn't hear it in this example, you will hear it if we oppose them directly.
[o] [ɔ][o] [ɔ][o] [ɔ][o] [ɔ][o] [ɔ]
We produce the closed o by rounding the lips like kissing the air. In opposite to the open o, where the chin moves downwards, in case of the closed o the chin doesn't move and the mouth remains almost closed.
If the o is in the unstressed syllable it is pronounced like a u.
(If you have problems to remember what is open and what is closed, think on open. The o in open is very similar to the open o and that is easy to remember. The open o is almost pronounced like the o in open. That's easy to remember.)