The Ohio State League was a minor league baseball league that operated in numerous seasons between 1887 and 1947, predominantly as a Class D level league. League franchises were based in Indiana, Kentucky, Michigan, Ohio and West Virginia.
 The first "Ohio State League" was a baseball league in 1887 before changing names to the Tri-State League. The Kalamazoo Kazoos won the first league championship. The league played in the 1889 and 1898 seasons.
 The first 20th century Ohio State League started in 1908 as a class D loop and ran through 1916.
 Another Ohio State League was formed in 1936 and ran through 1947 except for a break in '42-'43 due to World War II. In 1945, the OSL was one of just four class D circuits in operation, yet it was one of the first of dozens of leagues to collapse after World War II.
 The class D Ohio–Indiana League was formed in 1948 with eight teams, with seven of the eight cities coming from the Ohio State League. Six teams made their home in Ohio and the remaining two where from Indiana. This league lasted through
1951.
 The Portsmouth Cobblers won four Ohio State League championships.
 Baseball Hall of Fame member Ed Delahanty played for the 1887 Mansfield team.
 1887 Ohio State League
  Stubenville disbanded June 29; Canton entered the league August 4; Akron disbanded September 3.
 1889 Ohio State League
 Newark disbanded September 16; Tiffin disbanded September 18
 1898 Ohio State League
  Zanesville entered the league May 30; Conshohockten disbanded June 2 The league disbanded June 5 due to poor weather and Sunday baseball restrictions
 1908 Ohio State League
 Springfield moved to Portsmouth June 16.
 1909 Ohio State League
  Newark and Lancaster both disbanded August 23.
 1910 Ohio State League
 1911 Ohio State League
 Newark moved to Piqua June 22.
 1912 Ohio State League
 Marion moved to Ironton July 15.
 1913 Ohio State League
 1914 Ohio State League
 Ironton disbanded July 5; Newport-Paris disbanded July 5; Huntington and Maysville both disbanded July 22.
 1915 Ohio State League
 Chillicothe moved to Huntington July 13; Huntington moved to Maysville July 19.
 Playoff: Portsmouth 4 games, Maysville 1.
 1916 Ohio State League
 Huntington & Frankfort disbanded July 16; Charleston moved to Chillicothe July 13; Lexington disbanded July 16 The league disbanded July 19.
 1936 Ohio State League
 Mansfield and New Philadelphia both disbanded May 26.
 1937 Ohio State League
 Sandusky moved to Marion June22.
 1938 Ohio State League
  Playoff: Freemont 3 games, Fostoria 0.
 1939 Ohio State League
  Playoff: Lima defeated Tiffin in a one-game playoff for fourth place. Freemont won 2nd place by coin toss. Findley 2 games, Fostoria 0; Lima 2 games, Fremont 1 Finals: Lima 4 games, Findlay 3;
 1940 Ohio State League
  Playoff: Lima 3 games, Mansfield 0; Findley 3 games, Tiffin 1 Finals: Lima 4 Games, Findley 2.
 1941 Ohio State League
 No League play 1942 & 1943 due to WW II.
 1944 Ohio State League
 Playoffs: Newark 3 games, Lima 1;Middletown 3 games, Springfield 2 Finals: Newark 4 games, Middletown 2.
 1945 Ohio State League
 Playoffs: Middletown 4 games, Springfield 2; Zanesville 4 games, Lima 2 Finals: Zanesville 3 games, Middletown 2.
 1946 Ohio State League
 Playoffs: Zanesville 4 games, Richmond 0; Springfield 4 games, Newark 2. Finals: Zanesville 4 games, Springfield 2.
 1947 Ohio State League
 Playoffs: Zanesville 4 games, Springfield 0; Marion 4 games, Muncie 1; Finals: Zanesville 4 games, Marion 2.
 See the Ohio–Indiana League for 1948-51
 The Encyclopedia of Minor League Baseball; Second Edition
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