Who or what agency(s) is/are responsible for adobting the current
names we have for our states in the US. Like in 1915,
Okeechobee,Florida was founded. But it had been named Tantie for some
time by the locals.

Dear clubknowledge-ga,

The body responsible is the U.S. Board on Geographic Names ?BGN?. It
is a Federal body which was created in 1890 and established in its
present form in 1947. Its task is to maintain uniform geographic name
usage throughout the Federal Government.
You can find out more on their web site.
http://geonames.usgs.gov/

This is a link to their domestic page.
http://geonames.usgs.gov/domestic/index.html

This in turn links to the Geographic Names Information System (GNIS).
You can search on this for Okeechobee. The record tells us that a
decision was made in 1917 by the BGN, to recognise the name of
Okeechobee. Tantic is recognised as variant name.

?Board on Geographic Names Decisions
Feature Name 	Decision Year 	Authority 	Decision Type
Okeechobee 	    1917 	Board Decision 	Official?
http://geonames.usgs.gov/pls/gnispublic/f?p=121:3:6598889383040343659::NO::P3_FID:288073

I assume that the document archives will contain the relevant
paperwork as to how this decision came about.
http://www.archives.gov/research/guide-fed-records/groups/324.html

The wikipedia site below suggests that ?the names of Post Offices have
historically been used to back up claims about the name of a
community.?


This is a link to a 52 page document which describes BGN?s principles,
policies, and procedures.
http://geonames.usgs.gov/docs/pro_pol_pro.pdf

Other sources and background reading.
http://www.loc.gov/loc/lcib/0002/name.html

http://pittsburgh.about.com/library/weekly/aa081201b.htm

http://en.wikipedia.org/wiki/United_States_Board_on_Geographic_Names


I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you
answerfinder

Search strategy
"city names " us decision led me to a reference to the BGN. Then
search for their web site.