This is a kind of sound shift known in Japanese as 音便{おんびん}. The Japanese Wikipedia article has some good information, if you can read Japanese.

Historically, all the ''-i'' adjectives used to end in ''-ki'' for the attributive form (the form used when directly modifying a noun), and end in ''-ku'' for the adverbial form. "Good dog" would thus be よきいぬ, and "well done" would thus be よくした. In the Heian period, adjectives underwent two kinds of 音便{おんびん}, so-called イ音便{おんびん} and ウ音便{おんびん}, where adjectives lost the //k// sound in the attributive and adverbial forms, resulting in よいいぬ and ようした.

The イ音便{おんびん} has persisted in the modern language, whereas the ウ音便{おんびん} reverted back in eastern Japanese from some time around the Muromachi period (or, alternatively, ウ音便{おんびん} never manifested in eastern Japan and that evidence only appears in the textual record from around this time). I believe that Kansai-ben still exhibits ウ音便{おんびん}, and ウ音便{おんびん} does still persist in nationwide-standard 普通語{ふつうご} in certain expressions, such as お早{はよ}う (equivalent to お + 早{はや}く: hayaku → hayau → hayou) or おめでとう (equivalent to お + 愛{め}でたく: medetaku → medetau → medetou).

This sound change is historical and regular, both the disappearance of the //k// in adjective forms, and the shift from //au// to //oː//. The vowel shift also occurred in verbs, particularly in the volitional or presumptive form. This was originally -amu for 四段{よだん}活用{かつよう} or "quadrigrade verbs", composed of -a as the 未然形{みぜんけい} or incomplete form of the verb stem + -mu as the auxiliary verb ending expressing the presumptive or volitional. This -amu later lost its //m//, becoming -au, and this then underwent the regular vowel shift to -ou.

This is a kind of sound shift known in Japanese as 音便{おんびん}. The Japanese Wikipedia article has some good information, if you can read Japanese.

Historically, all the -i adjectives used to end in -ki for the attributive form (the form used when directly modifying a noun), and end in -ku for the adverbial form. "Good dog" would thus be よきいぬ, and "well done" would thus be よくした. In the Heian period, adjectives underwent two kinds of 音便{おんびん}, so-called イ音便{おんびん} and ウ音便{おんびん}, where adjectives lost the //k// sound in the attributive and adverbial forms, resulting in よいいぬ and ようした.

The イ音便{おんびん} has persisted in the modern language, whereas the ウ音便{おんびん} reverted back in eastern Japanese from some time around the Muromachi period (or, alternatively, ウ音便{おんびん} never manifested in eastern Japan and that evidence only appears in the textual record from around this time). I believe that Kansai-ben still exhibits ウ音便{おんびん}, and ウ音便{おんびん} does still persist in nationwide-standard 普通語{ふつうご} in certain expressions, such as お早{はよ}う (equivalent to お + 早{はや}く: hayaku → hayau → hayō) or おめでとう (equivalent to お + 愛{め}でたく: medetaku → medetau → medetō).

This sound change is historical and regular, both the disappearance of the //k// in adjective forms, and the shift from //au// to //oː//. The vowel shift also occurred in verbs, particularly in the volitional or presumptive form. This was originally -amu for 四段{よだん}活用{かつよう} or "quadrigrade verbs", composed of -a as the 未然形{みぜんけい} or incomplete form of the verb stem + -mu as the auxiliary verb ending expressing the presumptive or volitional. This -amu later lost its //m//, becoming -au, and this then underwent the regular vowel shift to -ō.