The O3 visa is for the spouse and unmarried dependents of an O1 visa and O2 visa holders, which allows them to enter the U.S. and reside with their family. While the spouse and dependent of the O1 or O2 holder are in the United States, they are allowed to travel outside of the U.S. on short trips and are authorized to reenter. Also, an O3 visa holder is allowed to study part time or full time while in the U.S. making it flexible for the dependent on continuing their education within the U.S.;
Futhermore, there can be some disadvantages while holding a O3 visa status within the U.S. Even though, the spouse or unmarried dependent can live in the U.S they are not allowed to work to help support their family. Also, period of time the O3 visa holder is only allowed to stay in the U.S. for no longer than the O1 or O2 holder can stay, which means the O3 holder may stay in the period necessary for the O1 visa holder to complete the event or activity, not exceeding three years.
An O3 visa is quite simple to obtain as long as you are a spouse or unmarried dependent of an O1 or O2 holder.