How does Oedipus complex fit in the evolutionary theory?
It doesn't.  The Oedipus complex is one of Freud's theories to explain human behavior, not something that is endorsed by the fields of psychology or psychiatry.  Freud had a lot of interesting theories, some of which remain core concepts in psychology, and some of which have been marginalized or rejected.

There are some professionals who feel it has merits for consideration, to one degree or another, but if you look at publications by the APA (American Psychological Association) and Psych.org (American Psychiatric Association), you won't see in their practice guidelines.

APA: http://psycnet.apa.org/journals/pst/32/4/535/

Wikipedia: http://en.wikipedia.org/wiki/Oedipus_complex#Criticism



If you're asking how might it fit with evolution if it were to be true, that's more of a philosophical question - which perhaps is the better angle from which to approach the question.  

In "survival of the fittest" - "fittest" is better described as "those who most successfully reproduce and produce offspring that are also highly successful in reproducing."  

Reproducing with one's immediate family results frequently in severe birth defects, the opposite of "fit."  Those who want to reproduce with their mother but don't, and reject all other mates, will not reproduce.  Not "fit."  The only possibility similar to the oedipal complex is that males choose mates that are like their mothers.  

What is true is that we often marry someone similar to our parent - but it is not a result of an actual oedipal complex.  Freud wrote the oedipal complex in part to try to explain this phenomenon.  But there are better explanations - we are attracted to that with which we are most familiar, to that which makes us feel comfortable, to that which makes us feel warm and fuzzy.  Our definition of beauty is largely shaped by family faces - because that is what you are surrounded with as your mind develops.  Etc etc.