Complex sentence: The Northern Oblast included 54 districts (including three districts of the Nenets Autonomous Okrug) and was in 1937 split into Arkhangelsk and Vologda Oblasts.
Split into simpler sentences: The Northern Oblast included 54 districts, three of which were in the Nenets Autonomous Okrug. In 1937, the Oblast was split into Arkhangelsk and Vologda Oblasts.