The Quite Reverend Mightily-Praiseworthy-Are-Ye-Who-Exalteth-Om Oats is an Omnian missionary priest who first makes an appearance in Carpe Jugulum where he found himself entangled in a war with the Vampires from Überwald who were trying to take over Lancre. Needless to say he goes by the shorter sobriquet although his full name is evidently shorter in Omnian. He came to Lacre from Ohulan because he was asked to fill in for the injured Brother Perdore at the naming ceremony for Princess Esmerelda Margaret Note Spelling of Lancre. Unfortunately he is responsible for her unusual name because Queen Magrat is so insistent that the same mistakes not be made with her daughter that were made when she was named (Magrat instead of Margaret) that she writes the name and instructions down for him. He becomes so nervous because of Nanny Ogg's presence that he read aloud the instruction as part of the name - an act that cannot be undone. Evidently this is not the first such mistake in the naming of royal babies as Nanny Ogg cites a past ruler of Lancre, King My-God-He's-Heavy the 1st. Oats redeems himself though by giving heroic assistance to Granny Weatherwax in the fight to evict the vampire Count de Magpyr and his family, whom King Verence had invited for the ceremony, not realizing that by inviting a vampire he was welcoming him to stay permanently. Oats is very similar to Agnes Nitt in that he possessed a divided personality - his penchant for asking questions contradicts his training to accept things on faith. This trait helps to make him almost immune to the vampyres' mind-control techniques, a feat that even the four Lancre witches (excepting Perdita) can't perform. Rev. Oats is an unusually sympathetic churchman for the Discworld: not hypocritical, a true believer, but also prepared to burn his beloved Book of Om in the service of mankind.
Over the course of the Discworld series, Mightily Oats grows in faith and might. He appears in Unseen Academicals as an early protector and mentor of the otherwise loathed and persecuted Mr Nutt. He preaches forgiveness, and Forgiveness is literally the name of the double-headed battleaxe that he carries with him through the fearful places of the world: faith being a tangible thing to Pastor Oats. Mr. Nutt returns to the Evil Empire to rescue what is left of his brethren with Pastor Oats.
Pastor Oats is mentioned in I Shall Wear Midnight by an Omnian priest (Pastor Egg) who tells Tiffany Aching that in his Testament from the Mountains, Oats said that witches embody the best ideals of Brutha the prophet (the main character in Small Gods).
He reappears in The Science of Discworld IV: Judgment Day to make a reasoned case at the trial hearing concerning ownership of the Roundworld, and is much altered from his appearance in Carpe Jugulum. It is clear that Vetinari holds him in high esteem, and his case swings the day against the Church of the Latter-Day Omnians.
Mightily Oats' name resonates with that of Titus Oates, the seventeenth-century anti-Catholic rabble-rouser and perjurer. Pratchett's Oats makes a nice contrast to the Roundworld Oates however as the latter was voted the third-worst Briton of the millennium in a recent poll whereas Pratchett's Oats is a force for good. The name also resonates with the expression, "Mighty oaks from little acorns grow" meaning appropriately that all great things start out small. The Puritan founder of the Barebone's Parliament of 1653, Praise-God Barebone (or Barbon), gave his eldest son, Nicholas, the unusual middle name of 'Christ-had-not-died-for-thee-thou-hadst-been-damned' so Pratchett is also likely to have been thinking of this when he gave Mightily Oats his full name. The Barebone Parliament was the predecessor of Oliver Cromwell's Protectorate.