Adding 으 to predicates stems
Some suffixes change based on whether a word has ends in a consonant or vowel. Many suffixes will add the syllable 으 if the word it attches to ends in a consonant. For example, the honorific suffix will become when added to a predicate stem that ends in a consonant.
The predicate stem of 괜찮다 ends in a consonant ㅎ so when combined with the honorific suffix 시, a 으시 will be placed after the predicate stem. Note that becomes in the polite speech level.
?Are (you) ok?
In contrast the the predicate stem for 돌아오다 does not have a 받침, so when combined with the honorific suffix 시 the predicate becomes 돌아오시.
?When does grandfather return?