How to create diacritical characters with the Dvorak keyboard on Mac?
Diacriticals for vowels and some consonants are created using the "dead key" system.

First you type the "dead key" by holding down two keys at the same time; for example, Option-E, which produces "´". Then immediately after that you let go of the two keys and then type the vowel that you want the accent to appear above. So in this example, to get "é", you type Option-E, let go, then type "e".

If you want "á", then you first type Option-E, then let go and type "a".

If you want "í", you first type Option-E, then let go and type "i".

Here are the dead keys in the Dvorak layout, highlighted in orange.