In a discussion at TypeDrawers, Greg Hitchcock (from Microsoft) shares a bit of the history regarding OS/2 table’s name in the TTF font format:
Because the design of fonts between OS/2 and Windows was very similar (the same folks at Microsoft did most of the graphics for both OS/2 and Windows – with some input from IBM based on their FOCA values) we decided to consolidate the OS/2 and WIN tables into just one table – OS/2. This is why the spec says “…a set of metrics that are required by OS/2 and Windows.” The parting with IBM occurred later in 1990. Microsoft had already made enough fonts using the OS/2 table that we decided it would be too expensive to rename the table to the WIN table.
Ultimately the OS/2 table has become somewhat of a catch-all for additional bits of data, which is why we are now on the 6th version of the table.