|This section uses algebraic notation to describe chess moves.|
He was four-time Czech Champion (1927, 1928, 1938, and 1944). In 1919, he took 2nd, behind František Schubert, in Prague (Czechoslovak Chess Championship). In 1925, he tied for 3rd–4th in Paris (Alexander Alekhine won). In 1927, he won in Česke Budějovice (CSR-ch). In 1928, he won in Brno (CSR-ch). In 1933, he won at Prague (the 10th Vaclav Kautsky Memorial). In 1935, he took 4th in Bad Nauheim (Efim Bogoljubow won).
In 1935, he took fourth place in Łódź (Savielly Tartakower won). 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 (Teplitz Schönau). In 1938, he won in Nice. In 1938, he tied for first with Hermann in Prague (CSR-ch).
Karel Opočenský played for Czechoslovakia four times in the Chess Olympiads.
- In 1931, he played at fourth board in 4th Chess Olympiad in Prague (+7 –2 =4).
- In 1933, he played at fourth board in 5th Chess Olympiad in Folkestone (+10 –0 =3).
- In 1935, he played at second board in 6th Chess Olympiad in Warsaw (+5 –4 =6).
- In 1939, he played at first board in 8th Chess Olympiad in Buenos Aires (+8 –5 =4).
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 8th Chess 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 (Bohemia and Moravia-ch). In 1941, he drew a match with Foltys in Prague (+4 –4 =4) and took seventh in Trenčianske Teplice (Foltys won). He also placed 13th in the Munich 1941 chess tournament (Europa Turnier), the event being won by Gösta Stoltz. In 1942, he tied for fourth-fifth in Prague (Duras Jubileé) behind joint winners, Alekhine and Klaus Junge. In 1943, he took 3rd in Prague (B&M-ch; Zita won). In 1944, he won in Brunn (B&M-ch).
After the war, he played in several international and local (Czechoslovakia) tournaments. In 1945, he tied for second-third, behind Emil Richter, in Prague. In 1946, he took 4th in Ostrava (CSR-ch; Luděk Pachman won). 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 (CSR-ch, Ladislav Alster won).
Opočenský is also known as a theoretician. There are two opening variations named after him: the Opocensky Variation in the Grünfeld Defence (1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.e3 Bg7 5.Nf3 O-O 6.Bd2) and the Opocensky Variation in the Sicilian Defence (1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Be2).
Awarded the IM title in 1950, he became an International Chess Arbiter in 1951.