Answer (12). The Cairo Geneze, by Paul Kahle, published in London says, “Not before 1100 was an o added to the word hwhyand this seems to indicate the pronunciation [Adonay]” (The Translations of the Bible, chapter 3, pp. 172-173, footnote 4).
It was a vowel sign for the letter o that was put in the middle of the Tetragrammaton. This led to the erroneous “Jehovah.” The Lexicon for the Books of the Old Testament says: “The wrong spelling Jehovah occurs since about 1100.” It then offers arguments in favor of Yahweh as “the correct and original pronunciation” (Koehler and Baumgartner, 1951 ed., vol. 1, p. 369, col. 1).
Because early Christians were not Hebrew scholars, they did not understand that the Tetragrammaton was pointed with the vowels for AdOnAY. Scholars maintain that the letter o or u is a vestige of this Rabbinical practice.
This technique was popular where the name why formed the beginning of a personal name, for example ucwhy (Yahshua), which was altered to Yehoshua by vowel pointing through the diacritical marks above and below the Hebrew letters (see subheading “How Did ‘Yahshua’ Become ‘Jesus?’” and request the ministudy, Spelling the Sacred Name: V or W?)