Many Texas organizational names include “Heart of Texas,” with “H.O.T.” added. There is no one organization that started all this, except perhaps the song “Deep in the Heart of Texas” (1941). The acronym “H.O.T.” was used by at least the early 1990s.


Wikipedia: Deep in the Heart of Texas
“Deep in the Heart of Texas” is an American popular song elaborating on the merits of the state of Texas. The song has become one of many de facto Texas state songs.

The 1941 song features lyrics by June Hershey and music by Don Swander. The song was first recorded by Perry Como with Ted Weems and His Orchestra on December 9 of that year for Decca Records in Los Angeles, California. It was a single release on the flip side of the song “Ollie Ollie Out’s In Free.” “Deep in the Heart of Texas” spent five weeks at the top of Your Hit Parade in 1942.
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Lyrics
The song alternates an appealing attribute of Texas (followed by four claps in quick succession), with the line “deep in the heart of Texas”. For example:

The stars at night, are big and bright, (clap, clap, clap, clap)
deep in the heart of Texas.

This Dog’ll Really Hunt:
An Entertaining and Informative Texas Dictionary
by Wallace O. Chariton
Plano, TX: Republic of Texas Press
1999
Pg. 132:
H.O.T. Heart of Texas, frequently seen on signs in and around Waco.

Google Groups: austin.personals
Newsgroups: austin.personals
From: (Harold Wellbaum)
Date: 1 Mar 1995 19:46:02 GMT
Local: Wed, Mar 1 1995 3:46 pm
Subject: H.O.T. Affair

In search of a discrete Lady in the Heart Of Texas (Austin, Waco, DFW area) with similar. 

Google Groups: rec.music.a-cappella
Newsgroups: rec.music.a-cappella
From: (Charlie Alvarez)
Date: 25 Mar 1995 05:16:10 GMT
Local: Sat, Mar 25 1995 1:16 am
Subject: Re: English BBS Arrangements

Lead - Heart Of Texas (H.O.T.) Chorus