Short answers, pulled from the story.
Old High German is the earliest stage of the German language, conventionally dated from around 500 or 750 to 1050. It encompassed the West Germanic dialects of the Upper and Central German territories that had undergone the Second Sound Shift, separating them from all other Germanic languages including English and Low German.
The Second Sound Shift, also called the High German Consonant Shift, was a set of consonantal changes that transformed inherited voiceless plosives into fricatives and affricates, likely beginning in the 6th century and complete by 750. It is the defining feature of Old High German, distinguishing it from Low German, Dutch, and English, which retained the older sounds.
The Abrogans, a Latin-Old High German glossary dated variously between 750 and 780 and probably from Reichenau, is generally regarded as the oldest surviving Old High German text. The 8th century Merseburg Incantations are the only surviving remnant of pre-Christian German literature.
Old High German never developed a supra-regional standard because the administrative and church language of the Frankish empire was Latin, not German, leaving no institutional pressure toward a unified form. Every text was written in a particular monastery dialect, leading scholars to call these varieties Klosterdialekte.
Notker Labeo was a scholar at St. Gallen who worked toward the end of the Old High German period and is counted among the greatest stylists in the language. He developed a systematic orthography for Old High German, and his death in 1022 was followed by a hundred-year gap in continuous texts that marks the transition to Middle High German.
Charlemagne's conquests by the last twenty years of the 8th century brought all continental Germanic-speaking peoples under Frankish rule, but both the administration and the Church used Latin as their language. With no political or ecclesiastical incentive, the individual dialects retained their own identities and no standard form emerged.