He was four-time Czech Champion. In 1919, he took 2nd, behind František Schubert, in Prague. In 1925, he tied for 3rd–4th in Paris. In 1927, he won in Česke Budějovice. In 1928, he won in Brno. In 1933, he won at Prague. In 1935, he took 4th in Bad Nauheim. In 1935, he took fourth place in Łódź. In 1935, he won in Luhačovice. In 1936, he took second place, behind Henryk Friedman, in Vienna. In 1937, he took second, behind Karl Gilg, in Teplice. In 1938, he won in Nice. In 1938, he tied for first with Hermann in Prague. Karel Opočenský played for Czechoslovakia four times in the Chess Olympiads.
He won individual gold and team silver medals at Folkestone 1933 and at Prague 1931, team bronze. When World War II broke out, Opočensky, Jan Foltys, and František Zíta were playing for the Bohemia & Moravia team in the 8thChess Olympiad in Argentina. They chose to return home, whilst team-mates Jiří Pelikán and Karel Skalička elected to remain in South America. In 1940, Opočensky took second, behind Foltys, in Rakovnik. In 1941, he drew a match with Foltys in Prague and took seventh in Trenčianske Teplice. He also placed 13th in the Munich 1941 chess tournament, the event being won by Gösta Stoltz. In 1942, he tied for fourth-fifth in Prague behind joint winners, Alekhine and Klaus Junge. In 1943, he took 3rd in Prague. In 1944, he won in Brunn. After the war, he played in several international and local tournaments. In 1945, he tied for second-third, behind Emil Richter, in Prague. In 1946, he took 4th in Ostrava. In 1946, he took fourth in London. In 1946, he tied for first with Daniel Yanofsky and Pachman, in Arbon. In 1947, he took fourth in Vienna. In 1949, he tied for third-sixth in Vienna. In 1949, he tied for fourth-fifth in Arbon. In 1956, he took third in Poděbrady. In 1951 and 1954, he was the chief arbiter for the World Chess Championshipmatches in Moscow, and also in the 10th Olympiad at Helsinki 1952, and in the second Candidates Tournament at Zurich 1953. Opočenský is also known as a theoretician. There are two opening variations named after him: the Opocensky Variation in the Grünfeld Defence and the Opocensky Variation in the Sicilian Defence. Awarded the IM title in 1950, he became an International Chess Arbiter in 1951.