It's nothing serious. That happens with a lot of different genera of orchids. Like Carl had said, unless the entire spike is blasting, there's nothing to worry about. Or if the signs are very obvious that there's something wrong, I wouldn't bother with something like this.
If there are other problems such as being kept too dry, a photo of the entire plant is necessary to see if that's actually the case.
Have you checked the roots?
Oncs tend to be a bit more difficult to mind the root health to, because most have fine roots.
Links to pics of healthy, living orchid roots, with growing green root tips:
http://www.aos.org/AM/Images/pest+di...snaildmage.jpg