w/ appears to have origin in the food industry some 70 years ago (according to this question).
To me it makes sense, as I first encountered it in video games where abbreviations, acronyms, and text-saving-slang are commonplace. Furthermore, while abbreviations usually have multiple letters (in written text, not physical or mathematical equations), single letter abbreviations can quickly become confusing, so I belive that this is the reason for putting a slash behind it, or possibly a bar above it.
RANT:
While I know that language changes all the time, I find it very unfortunate that this little fellow o/
and possibly his slightly more formal friend o7
have become synonymous with "nazi salute". First off, it's the wrong arm! And second off, what do you have against "man waving" and "man saluting"?
It must be very confusing for someone who uses this newer definition of o/ to visit the Elite:Dangerous forums.
EDIT: I'm very happy that I apparently am the only one who has met people who don't know the real meaning of o/ and o7. I feared that this was a widespread problem, but luckily it appears that I simply am a worrywart.