Should the first letter of a quote used to start a sentence be capitalized even if it is not capitalized in the original text?

When dealing with a direct quote that is a complete
sentence, you must capitalize the first letter, no matter where the quotation is placed
inside the sentence:

Ricardo noted, "If it rains
today, then tennis practice is
cancelled."

Notice that before
the quoted material I set up a framework--I provided background/contextual
information. Next, I offset my framework from the quote with a
comma, although I could also use a
colon. If I want to begin my sentence with the same
quotation to add variety to my writing, then my punctuation and capitalization still
stay the same:

"If it rains today, then tennis
practice is cancelled." Ricardo noted.

Now for
the fun stuff! If I wanted to only quote a word or a dependent clause (aka a fragment),
then I could also do it several ways:

The term
O.K. was originally a facetious spelling of the phrase "oll
korrect."

No capitalization needed there! Now,
if I want to start my sentence with the same quotation--which is not a complete sentence
and not capitalized originally--I must
capitalize:

"Oll korrect" is a facetious phrase
from which the term O.K. is derived.

Whenever
beginning a sentence, even if it is with a piece of quoted material,
always use a capital letter as a courtesy to your readers
so that they know you're beginning a new thought.