It seems likely the item to be asking about is not country but oil country...
"Oil Country" in OCTG could refer/allude to (a) a nation that produces oil; (b) a region called 'oil country', as outlined in the bulk of this answer eg the original Oil Country centered in Pennsylvania; (c) all such oil countries, in the sense of either a or b; (d) perhaps to some original company or market called Oil Country Tubular Goods; (e) anywhere where the oil industry is present, kind of like Boston Nation refers to fans of Boston sports teams no matter where they live; (f) something else.
I've emailed the American Petroleum Institute and asked. It's website says standardization of oil field equipment didn't begin to after World War I.
One definition of country is
A region, territory, or large tract of land distinguishable by features of topography, biology, or culture: hill country; Bible country.
American Heritage Dictionary
Thus, oil country refers to any "region, territory, or large tract of land" characterized by the presence of oil or petroleum (which literally means "rock oil") and the industries, services, goods, and indeed the wealth and even culture that develops from the extraction, collection, transportation and sale of crude oil.
In the USA, and I think in the English-speaking world, the original oil country (once referred to as the oil country because it was so specific & famous a region1) was that portion of northern and western Pennsylvania, etc, including the Oil Creek Valley (where once American Indians would collect surface oil), which saw
the Pennsylvania Oil Boom/Rush after Edwin Drake drilled the first successful well in 1859.
Since then, both in the USA and elsewhere, other oil countries have arisen, including parts of Texas and California. There's one in
Note that some place called oil country can also be called something else country if the same region has more than one defining characteristic: a place could be oil country, hill country, cattle country, beer country, redneck country, Willie Nelson country, etc., all at the same time. Yes, I'm still thinking of Texas.
Also, note that very often, banks, stores, restaurants, parks, zoos, pretty much anything, will call themselves after the characteristic of where they are located. So you can find Oil Country Federal Credit Reserve in Pennsylvania.
Thus, oil country refers first and foremost to a geographic location. Perhaps its usage in Oil Country Tubular Goods is allusion to one of these physical regions, maybe to all of them.
Somehow, and I don't know when, where, and why, the term Oil Country Tubular Goods (OCTG) came to be applied to certain goods used anywhere and everywhere in the petroleum industry.
The purported 1859 usage seems specious, some kind of scanning error. The language is not that of the mid-19th century and there is no other usage recorded in Google Books of the term until the early 1970s.
1 See, for example, "The Oil Country and Its People" in Magazine of Western History, 1885.