Thanks to Phyllis Blanchard ExtrOversion is the prominent spelling of the word in the United States today.
In her 1918 paper, “A Psycho-Analytic Study of August Comte” she writes:
“In order to understand the marked contract between Comte’s mental attitude during his early years and that of his later life, we must keep in mind Jung’s hypothesis of the two psychological types, the introvert and extrovert, – the thinking type and the feeling type.”
Not only did she change the spelling of the word, but she also changed the definition!