Singleton letters in direct speech

I am writing a scene where one of the characters screams, "H" and "O-H." I want to know the best to indicate that to the reader, that the distinct letters are being pronounced, not Oh, but O and H. I am also doubtful that "H!" might not be clear as the letter ‘H’ and might be taken as a mild interjection, like "hmmm." Hence I was considering "’H’!" but that looks just as messy and confusing. Also about O,H, should I present it as "O-H!", "’O’, ‘H!’" or is there some better convention? Any help is appreciated and this might just be a question on punctuation. I also want to emphasize the pronunciation of the letters, "H" as "Eechh," so is that a viable option or should I just describe that in the paragraph following the dialogue?