No, it was not a Low German dialect. It was what is called an Ingvaeonic dialect, most closely related to Old Saxon and Old Frisian. Old High German is what is sometimes called Istvaeonic (and the Franconian dialects that were to become Dutch is also part of that group). There is a bunch of important differences between Ingvaeonic and Istvaeonic, and both are quite, quite different from North Germanic.