Final answer:
Cicero outlined six parts of an oration in his work 'De Inventione': Invention, Arrangement, Style, Memory, and Delivery.
Step-by-step explanation:
The six parts of an oration according to Cicero can be found in his work De Inventione, which is often considered an essential piece for understanding classical rhetoric. The six parts as described by Cicero are: Invention (the process of developing arguments), Arrangement (organizing the arguments for greatest impact), Style (the choice of language for making the argument), Memory (the practice of memorizing the speech), and Delivery (the gestures, pronunciation, and rhythm used in presenting the speech). It's important to note that Cicero was a master at the art of public speaking and his guidelines influenced orators for centuries.