Holy cow, I wrote a book!
Last year, we learned that
the ANSI code page isn't actually ANSI.
Indeed, the OEM code page isn't actually OEM either.
Back in the days of MS-DOS, there was only one code page,
namely, the code page that was provided by the
original equipment manufacturer
in the form of glyphs embedded in the character generator
on the video card.
When Windows came along,
the so-called ANSI code page was introduced
and the name "OEM" was used to refer to the MS-DOS code page.
Michael Kaplan went into more detail earlier this year
on the ANSI/OEM split.
Over the years, Windows has relied less and less on the character
generator embedded in the video card, to the point where the
term "OEM character set" no longer has anything to do with the
original equipment manufacturer.
It is just a convenient term to refer to "the character set used
by MS-DOS and console programs."
Indeed, if you take a machine running US-English Windows
(OEM code page 437) and install,
say, Japanese Windows,
then when you boot into Japanese Windows,
you'll find that you now have
an OEM code page of 932.