If someone is excited, they will often put exclamation points at the ends of their sentences to indicated their excited tone of voice. If they say “Oh!” for instance, the exclamation point would indicated that the person is saying “Oh!” in an excited way, rather than simply saying “Oh,” with a comma after it, which would indicate that the present is somewhat disappointing.
“Oh,” said Violet, as she opened her present.
“Oh,” said Klaus, as he opened his.
“Oh,” said Sunny, as she tore open her shopping bag with her teeth.
“Pinstripe suits! I knew you’d be excited!” Esmé said.