Well, the class O felony does not exist (maybe it is a typo). But, if you tell me what the charges are, I can tell you the punishment. Also, the class U felony just means that the felony is "unclassified." Therefore, the punishment range is actually listed in the statutory section of the actual charge (I have heard lawyers get confused about this). So, class U felonies can have different punishments and you need to know what statute he is charged under - and that statute will tell you the sentencing range.
This advice is generic and not designed to substitute for consulting an attorney. Nothing in this response is designed to establish an attorney-client relationship with the requester.
As the other attorney said, Class U felonies are simply "unclassified." This means, for whatever reason, the General Assembly chose to punnish it differently than the Class 1-6 felonies we have in VA. There are a number of Class U felonies out there, and even an unclassified misdemeanor; to know what the punnishment is you have to refer to the statute. Take a look at the warrant and do a google search for the statute. You should be able to figure it out pretty easily. Or call a lawyer for assistance.