There are no known dangerous interactions between cocaine and olanzapine. Actually studies have been conducted to investigate its potential as a treatment for cocaine dependence (some studies concluding it is effective while others conclude it is ineffective, in contrast to some other
atypical antipsychotics, which do seem to be effective;
and compared to some
typical antipsychotics which seem to actually
increase potential for cocaine dependence).
Another study concluded olanzapine actually protects from acute cocaine toxicity (in rats;
PubMed-link).
If anything, it seems to be a good thing to combine olanzapine with cocaine use. Also, as gammagoblin pointed out, it might decrease the effects of the cocaine slightly. Although, according to
wikipedia, olanzapine seems to primarily affect serotonin receptors, and to a lesser degree dopamine receptors. So, its attenuation of cocaine's effects is probably really very small, compared to antipsychotics that primarily affect dopamine receptors.
One more thing you should know, if you don't already, is that people with a psychiatric condition are on average more prone to substance dependence, compared to the general population. And, as gammagoblin said as well, cocaine can increase psychotic symptoms.
So really keep your cocaine use to a minimum and do not compare yourself to friends who are not in a similar condition. Be extra careful and start with low doses to see how you respond.