What word follows "u"? "U" becomes "o" (or) before a word starting with "o" or "ho" since the "h" is silent. Hope this helps!

Cual es el contexto ?

"U" can be a way to say university too

As Amy told you. The "o" change for "u" when the next word begin with the vowel "o". Is similar than the use of "a" and "an" in English, but this case apply only to "o". Example: ¿Quién es mejor jugando fútbol Carlos u Oscar? (Who is better playing soccer Carlos or Oscar?) The same things happen with the "y" if you knew perhaps you undestand better. This change when the next word begin with "i". Example: Creatividad e ingenio son muy importantes para la generación de ideas (creativity and wit is very important for the generation of ideas). I hope this could be helful.