Grandpa is a foodie. Does not mean that he likes everything though.
He likes Crab but not Lobster
He likes Roti but not Naan
He likes Apricot but not Plum
Why is that?
He also said that his likes are related to him (also) liking the following words
Obituary
Quixotry
Opacity
Outbrag
Can you now figure it out?