This came up in another thread in the general discussion forum, and I thought I'd move it here.
Common fan theories say that Mitchell was the XO in "Where No Man Has Gone Before." And there's been much speculation just based on the pilot itself.
However, I recently read the first draft of the script by Samuel Peeples, which further cements what I've always asserted: Spock was the first officer, not Mitchell.
First, let's go by what's on screen.
It is clear that Spock is the first officer in "Where No Man Has Gone Before." He wears command gold, sharing the same emblem as Kirk on his uniform. He advises Kirk on the situation, giving him alternatives — strand Mitchell on Delta Vega or "kill him, while you still have time."
Now let's go to the source of the pilot — the script.
In the final draft of the script (dated July 8, 1965), Mitchell is listed as the "ship's senior helmsman" not first officer. Spock is listed as the "science officer."
However, in the first draft (dated May 27, 1965), the actual relationship between Spock and Mitchell is spelled out.
While Spock is still listed as science officer, Clark (not Gary) Mitchell is said to be:
Regardless of popular fandom theory, Mitchell at no time is referred to as Kirk's executive officer, whereas Spock is shown to act in that role throughout the episode.
Hence, Spock is the first officer in the second pilot.
Common fan theories say that Mitchell was the XO in "Where No Man Has Gone Before." And there's been much speculation just based on the pilot itself.
However, I recently read the first draft of the script by Samuel Peeples, which further cements what I've always asserted: Spock was the first officer, not Mitchell.
First, let's go by what's on screen.
It is clear that Spock is the first officer in "Where No Man Has Gone Before." He wears command gold, sharing the same emblem as Kirk on his uniform. He advises Kirk on the situation, giving him alternatives — strand Mitchell on Delta Vega or "kill him, while you still have time."
Now let's go to the source of the pilot — the script.
In the final draft of the script (dated July 8, 1965), Mitchell is listed as the "ship's senior helmsman" not first officer. Spock is listed as the "science officer."
However, in the first draft (dated May 27, 1965), the actual relationship between Spock and Mitchell is spelled out.
While Spock is still listed as science officer, Clark (not Gary) Mitchell is said to be:
He is superior to Kelso, and junior to Spock.

Regardless of popular fandom theory, Mitchell at no time is referred to as Kirk's executive officer, whereas Spock is shown to act in that role throughout the episode.
Hence, Spock is the first officer in the second pilot.