B or BO

We use "В" with the words beginnig with vowels. eg  - В океане

And "Во" for the words beginning with consonants. eg - Во дворе 

Irina, not so simple. For instance, I will say "В замке/в крепости/в магазине", but they all start with consonants. There is no strict rule. I have found similar question with good answers here: http://otvety.google.ru/otvety/thread?tid=3a5f8ea58e170c25&pli=1

It gives me a good general idea.

"О" is a  connecting vowel. For exemple, in french you speek: '' Quel musée cherche-t-il?'' - He searches for what museum?. Or in english you use '' There are...''. The last latters said, but ussually not said.

So, this object hasn't a rule.

В банке, во саду= в саду, в руке, во мне( but not  в мне), в огне(but not  во огне)