Available on open shelf in the Official Papers reading room.
International statistical yearbook O.LN per. 13 (1926-1941/42)
Monthly bulletin of statistics O.LN per. 10 (1921-1942)
Monthly summary O.LN per.8 (v.1:1920-v.8:1940)
Official Journal O.LN per.1 (v.1:1920-v.21(1940)
Also in full text via HeinOnline (search under title 'League of Nations Official Journal')
Proceedings and resolutions of the Assembly were published in the Special Supplement of the Official Journal 1920-1938.
Minutes of the Council were published in the Journal 1922-1939
General texts about the League of Nations can be found on the open shelves in the Bodleian Law Library in the international section at Internat 685