Since it's very easy to misread, it should be explicitly mentioned that the book is actually titled Racso and the Rats of NIMH (note order of 'cs' in the first word).
During the course of the story, we find out that the strangely spelt Racso is in fact Oscar backwards.
IIRC, he adopted this variation because he didn't much like his real name. Admittedly it has been quite some time since I've read the story, so I may be wrong there.