Werthead
Explorer
The Forgotten Realms first appeared in print in Dragon way back in the late 1970s. The first canonical mention of Waterdeep and Thay is in 1982 and the first mention of Baldur's Gate is in 1984, all long before Kara-Tur appeared in Oriental Adventures in 1985 and before it was formally added to the Realms setting in 1987 (with the Kara-Tur: The Eastern Realms boxed set followed in 1988, although they screwed up the scale and made Kara-Tur far too big, so it had to be scaled down in 1990's The Horde boxed set).
The history is a bit more involved than that. Gygax really wanted an Asian-themed sourcebook and pushed for Oriental Adventures for some considerable time (it was in the works for something like 3-5 years, which was an ice age compared to how fast some 1E and earlier books were created), which went through several iterations before settling on its final format under Dave "Zeb" Cook. As the book approached publication, TSR pulled off shenanigans that rotated Gygax out of the company. The original plan, that Kara-Tur would be located somewhere on Oerth, went out the window.
Interesting, Oriental Adventures only has a few pages of setting material right at the end mentioning things like Shou Lung only in very broad terms, and no map.
With Gygax gone, TSR staffers seemed to feel uncomfortable carrying on with Greyhawk setting material without him, and they also seemed to feel that Dragonlance was not a suitable replacement (too far from core D&D with no orcs or halflings, too many setting-specific species, the whole thing revolved around a metaplot and sales seemed to be well down on gaming material although the novels were still selling gangbusters). The first few Oriental Adventures-related adventures were likewise setting neutral.
In 1986, TSR bought the Forgotten Realms off Greenwood (who'd created them for home use in 1967, had been publishing fiction in them in local magazines and chapbooks in Canada since 1976 and been writing Dragon articles set there since 1979) and made the decision to disregard Greenwood's own expansive 1982-created world map (something he occasionally brings up as a source of mild irritation to this day) in favour of bolting on Kara-Tur to the eastern side of the Realms, early enough so it appears in the 1987 Old Grey Box. Around that time we also see Oriental Adventures line of adventures start to include Kara-Tur maps showing it on the eastern side of a continent (if it was going to be on Oerth and part of Oerik, by necessity it would have been on the western side).
People have been asking Ed to publish his original world map, which shows a completely different eastern side to Faerun, Laerakond (which did eventually appear in 4E, in a different context) to the west instead of Maztica, Anchorome as a massive island-chain to the north-west, and the mysterious continent of Arondron in the southern hemisphere, along with several other continents. He hasn't done that so far.
