Greg Cox wrote:
I admit that I cheated and added a female security officer to my next TOS book, but, yeah, Spock is clearly the only "alien" officer aboard the Enterprise in in the original series, given the way his Vulcan nature seems to be a constant source of interest and curiosity from the rest of the crew. One gets the distinct impression that Spock is regarded as something of a novelty by the otherwise human crew of the Enterprise.
On the other hand, various novels over the years have given the Enterprise
more alien crew during the TOS era -- the most recent one I can think of being A Choice of Catastrophes