This paper seeks to determine the developments of Old English/early Middle English ō in Middle English dialects, by making use of irregular spellings extracted from three corpora of Middle English dialect material, as well as from other sources of localised Middle English texts. I argue (a) that Northern Fronting of ō started before 1300 in the northernmost counties of England and spread southwards during the next 100–150 years, (b) that the Great Vowel Shift raising of ō started in the mid-to-late thirteenth century in the West Midlands and East Midlands and a little later in the South, and (c) that there is thus temporal overlap between Northern Fronting and the Great Vowel Shift. These long-vowel changes should therefore not be treated separately: if there was any ‘Great Vowel Shift’, it must include Northern Fronting, as well as other presumed earlier changes.