2779: Old English Sound Shift A to O Jul 30, 2024

Before the Great Vowel Shift in the early days of Modern English, there were many smaller sound shifts. The transition from [ɑ] / [æ] to [o] in Old English is one such example, that is less linear than it would at first glance appear. 

One of the key processes that led to the change from [ɑ] to [o] was breaking and diphthongization. In other words, a single vowel sound, here [ɑ], into a complex vowel sound. Eventually however, the reverse occurred and the breaking (diphthongs) merged into one vowel, but it was then realized as [o]. For instance, how this looked:

Haldan (to hold) became healdan, and later holdan. Other -old words, including gold, bold, fold, cold, and indeed old developed the exact same way from Old English.

The Old English word ‘cælf’ (calf) also underwent breaking to become "cealf," and eventually "calf" in Modern English. The way it reverted back is not relevant to this process, and was affected by a later shift. 

As was hinted at, the sound shift was not total—or else there would be no words with [ɑ] / [æ]—but occurred within particular linguistic environments, i.e. the surrounding consonants would influence [ɑ] / [æ] to break or not, such as before /ld/.