Opus means musical composition, so in the context of the movie the word makes some sense. But he never becomes rich and famous and ends up teaching for thirty years and Richard Dreyfuss goes from Richard Dreyfuss with dyed hair to Richard Dreyfuss with grey hair and a big fat gut. Nevertheless, Mr. Holland and his wife have a kid who doesn't produce enough family excitement so the producer and director decide to give him an affliction.
Afflictions are good, as every filmmaker knows, because they generate emotion and win Oscars.
In this case the affliction is deafness, which Mr. Holland doesn't learn to cope with until he miraculously realizes that while he's become a good teacher, he's also become a bad parent.