This is the 8588th most frequent Russian word.
окнийским
The term "окнийским" does not appear to be a standard Russian word. It may be a misspelling or incorrect form.
In this sentence, 'окнийским' describes something related to the Olympics, namely the subject the person chose.
Его окнийским предметом стала физика.
His Olympic subject became physics.
Here, 'окнийским' refers to the level or standard associated with the Olympics, describing the tier of competitions.
Окнийским уровнем соревнований оказался чемпионат страны.
The Olympic level of competitions turned out to be the country's championship.
In this usage, 'окнийским' designates a type of sport recognized or associated with the Olympics.