Decide in the following chapters whether the marked os are open or closed. In the case of a closed o, that doesn't exist in English, the lips are rounded, the chin doesn't move, the mouth is more closed. Try to pronounce offer, actually an open o, with rounded lips and without moving the chin and keep your mouth almost close, like kissing the aire. The result is not English, but very Portuguese. In the spelling the closed o is marked with an acento circunflexo, ô, and the open o with an acento agudo, ó.
Very often however the o doesn't have any accent and therefore the way it is to be pronounced can't be deduced from the spelling. There are rules that allows to deduce the spelling, but this rules are so complicated that they are useless from a didactical point of view.