I know its a few months since you posted this, but i thought i'd reply anyway. i would thoroughly recommend Asanka Local in Osu. it is a cafeteria style "chop bar" serving very good Ghanaian dishes at cheap prices. the low prices gives you the opportunity to try different types of food without worrying too much about cost.
i'm a Ghanaian living in Europe and I always make sure i eat at the Asanka on every visit.
next time I am in Ghana! We even stayed in Osu, but didn't make it to Asanka. Thank you for the reply, I'm sure it will help someone.
I loved Ghana. I really miss all the forms of fried plantains, and also this bag of sweet dark brown banana chips I got at the border of Togo and Ghana. I miss those so much. Now I'm in Perth and there are no plantains.