The first IOI was in 1989

In March 1989 at the “Future Development of International Olympiads for Youth” consultative UNESCO meeting, Professor Pavel Azalov presented reports titled “Introduction to the Olympiad in Informatics” and “Bulgarian Competitions on Informatics”.

“Bulgaria - The Homeland of International Competitions in Informatics for School Students”
Article by Acad. Prof. DSc. Peter Kenderov, Chairman of First IOI, 1989

Official Proceeding of the First IOI

The first IOI was in 1989

Document-ul de baza al olimpiadei din 1989:
http://www.math.bas.bg/talents/en/inf/International_Olimpiad_in_Informatics.pdf