The difference is great. "Usually" means "under normal conditions". It gas no connotation of frequency. "Often" means "frequently". Example: We usually take a flu shot once a year. That's not often at all, us it. We usually celebrate a centenary once every hundred years. That's not often at all!

They are synonyms. Q:"How often do you go to the movies? A: "Usually, I go to the movies twice a year." "I usually take the bus." "Oftenly= usually" just add a ly to the end and you can use one or the other.