You can substitute orange extract for orange peel in equal parts by volume without changing the recipe. For example, if your cake calls for 1 teaspoon of orange zest, add 1 teaspoon of extract to the wet ingredients. If you're making frosting, add the extract after you whip the butter. You can also substitute 1 teaspoon of orange liqueur for 1 teaspoon of orange peel.
Making Your Own Extract
You can substitute orange zest for orange extract with the addition of a liquid, or make your own extract with alcohol. Mince enough orange zest to make at least 1/4 cup and place it in a glass jar. Add just enough triple-distilled vodka to cover the zest and lid the jar. Let the alcohol extract the oil from the zest for two weeks; shake the jar every day. Pour the extract through a sieve lined with cheesecloth and store it in a jar.