I always thought that if it ended in ος it would be masculine
No, while most nouns ending in -ος are masculine, there are also feminine and neuter ones with this ending. For example, η οδός (the street), η περίμετρος (the perimeter), το μέρος, το δάσος (the forest), το έδαφος (the ground).
Up until this lesson the nouns have had the genders listed with them in the hints. When coming across the words for the first time this is invaluable - in any classroom learning of languages that I've experienced it is always stressed that it is better to learn the gender with the noun. In this lesson the genders don't seem to be present.