when I type on OB:
i push enter and it erases what i type, whats the matter 
Our school uses Mac Osx 10.5 and happens on both computers
Im not sure, but try typing in your new OB name and then clicking in the 3D view or some other non active area?
If you’re on a Mac, why are you using Enter? Did you try Return?
Are you sure you have the right OB: field? You should be in:
The Link and Materials panel of the F9 Editing Context Buttons Window, or;
The Object and Links panel of the F7 Object Context Buttons Window.
There are other OB: fields that link objects together. These require an exact, case-sensitive match with another object, or they’ll behave exactly as you described. They also don’t change the name of any object.