The Christian Mirror, Portland, Maine, 9 April 1840, p. 3: "In Exeter, Capt. Nathaniel Oak, aged 88 years 10 months. The deceased arose in usual health, walked from his residence to the meeting house .. and had been seated but a few moments, when he suddenly fell and expired."
Image · dated 9 April 1840 · source: The Christian Mirror, 9 April 1840, via GenealogyBank (screenshot).

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Keep in mind: Confirmed as printed; the age digits read 88 at zoom (they could be 83), and no date of death is given. The notice was copied from the Dover Herald, which was not found. A back-calculation from 88 years 10 months fits a 15 March 1840 death and a May 1751 birth.