The Oviedo post office was established on 13 March 1879 as reported by the United States Postal Service.
Oviedo was originally part of Orange County. It wasn't until 1913 that Seminole County was established.
The first postmaster was Andrew Aulin who served from 13 March 1879 to 20 June 1882.
Shown here is a cover with handwritten (most likely in Andrew Aulin's hand) town and date on two lines: Oviedo Fla, 4 March 1880. Also by hand is the "X" to cancel the postage. Addressed to W. A. Stillman M. D., Saratoga Springs, New York. Earliest known Oviedo cover. (Scott U163, Design U45, Die 2.)
The arrival handstamp on the back, SARATOGA SPRINGS, N. Y., MAR 9 3PM, demonstrates travel time of 5 days. Probably traveled from Oviedo to the newly established Lake Jesup post office, then by steamer to New York.
The USPS web site at: htps://about.usps.com/who/profile/history/postmaster-finder/post-offices-by-est-date.htm can be used to search for post offices, postmasters, and much more.