The Chicago Manual of Style warns careful writers to avoid confusing the words oration and peroration:
A peroration, strictly speaking, is the conclusion of an oration (speech). Careful writers avoid using peroration to refer to a rousing speech or text.
In its rhetorical sense, a peroration is the concluding part of a speech intended to sum things up and rouse the audience to some action.
He…[concluded] his speech with a peroration whose purpose was to remind the audience that he was among the few Republicans with a plausible shot at occupying the White House.
Toward the end of the speech, King departed from his prepared text for a partly improvised peroration on the theme “I have a dream”…
The expression “a rousing peroration” in the sense of “a fiery speech,” is to be avoided, if for no other reason than it’s a cliché.
In other contexts, however, the word peroration has been used since the 15th century to refer to a whole speech or utterance.
Shakespeare used peroration in 1591 as a synonym for discourse:
Nephew, what means this passionate discourse, This peroration with such circumstance? For France, ’tis ours; and we will keep it still. –Henry VI, Part II, I.i, 111.
Later writers, including Harriet Beecher Stowe, Sinclair Lewis, Mark Twain, and F. Scott Fitzgerald, all used peroration to refer to comments other than the concluding part of an oration. In the following example from recent news item, peroration refers to a speech and not to the concluding part of a speech:
When the de Blasios arrived a little after 10, the candidate gave a brief peroration to the gathering outside, which surely numbered more than one hundred.
Certainly it would be an error to speak of “the perorations of Pericles” if what is meant are “the orations of Pericles.” But while the use of peroration as a synonym for speech might be justly considered stilted diction, it’s not an occasion for ridicule. Calling a politician’s whistle stop speech a peroration is no worse than using decimate to mean “to kill an indeterminate number of people” because the word’s “real” meaning is “to kill one in ten.”