[Opinions] It's referred to as the "cot-caught merger" (m)
I might have been a little generous when I said "most" because it is on a scale, but more than 50% of Americans merge them. From Wikipedia:
"The cot-caught merger (also known as the low back merger) is a phonemic merger, a sound change, that occurs in some varieties of English. The merger occurs in some accents of Scottish English and to some extent in Mid Ulster English but is best known as a phenomenon of many varieties of North American English.
The sound change causes the vowel in caught, talk, and small to be pronounced like the vowel in cot, rock, and doll, so that cot and caught, for example, become homophones, and the two vowels merge into a single phoneme. The change does not affect a vowel followed by /r/, so barn and born remain distinct, and starring and warring do not rhyme.
The presence of the merger and its absence are both found in many different regions of the continent, and in both urban and rural environments.
According to Labov, Ash, and Boberg, the merger does not generally occur in the southern United States (with exceptions), along most of the American side of the Great Lakes region, or in the "Northeast Corridor" extended metropolitan region from Providence, Rhode Island to Baltimore. Areas that it occurs include:
* Canada
* Boston (see Boston accent)
* Northeastern New England
* the Pittsburgh area (see Pittsburghese)
* The Western United States
* Due to an apparent spread of the merger towards the center of the United States (from both the western and eastern states), portions of the Midwest also feature the merger:
o Illinois
o Indiana
o Iowa
o Minnesota
o Missouri
o Ohio"