How to accent the 'a'/second syllable in Oscar?
Solution 1:
Presumably Oscar is not English.
You could indicate this and the pronunciation by using an accent: Oscár. Accents are not used in English, and can be used in other languages to indicate stress. A quick Google search yields Oscárs from Spain, Sweden, Romania and Hungary.
However, even with this non-English treatment, it might also be advisable to have the incorrect "English" pronunciation corrected by Oscár himself or another character soon after his introduction to the story.