SELECT id, haslo, znaczenie, data FROM sennik WHERE  (haslo LIKE 'O%' OR haslo LIKE 'o%' OR haslo LIKE 'Ó%' OR haslo LIKE 'ó%' ) ORDER BY haslo ASC

Illegal mix of collations (latin2_general_ci,IMPLICIT) and (utf8_unicode_ci,COERCIBLE) for operation 'like'