Old High German was influenced strongly by Latin, its literature and vocabulary. This was the first difference occurred between High and Low German. But internal processes were much more important. First of all, unvoiced fricatives p, t, k became unvoiced spirants pf, ss, ch, while they were preserved in Saxon. So Low German water means High German Wasser. The narrow long vowels became diphthongs in High German: e.g. îs > eis (ice). Vice versa, short vowels were prolongated in open syllables: name > náme.
Old High German was productive and developed very quickly, already in the 12th century its morphology changed a lot. So the Middle High German epoch began, but that is already the history of the modern German language.