why was osvaldos testimony important

As a part of the robbery he isn't exactly a great character witness. He took part as a look out, and testifies that rather than do what he did for the money, he did it because he was intimidated by and afraid of Bobo.

Regardless, we're talking about two different people here. Steve has rarely been in trouble, Osvaldo has been in trouble his whole life. He thinks nothing of beating his girlfriend (she's pregnant to boot).....

Sorry, Osvaldo's testimony was also important because he told of King's being the murderer, and that Steve was supposed to be the lookout.