Let's see.

native 8086 binaries [os? we don't need no os!]
basic <- 8 bit OS
ibm dos 3.3
ms-dos 5.0
windows 3.10
os/2 2.1
pc-dos 6.1
os/2 3.0
pc-dos 6.3
os/2 4.0
pcdos 7.0
pcdos 2000
windows 98
windows 2000

xp lasted a day or so, when i booted w2k in a vm, it just had to go on.
vista was a joke, win7 doubly so. (i have 2 copies of ultimate, never installed )

Oldest machine I have used was Atari and ZX Spectrum+

In the PC world, I used MS-DOS 7.1, Windows 3.11 for Workgroup, Windows 98 on real machines.

havent gone back that far but the os seems ancient any way, unstable, slow... But alas I owned windows me. Never use this as a main os

owned: Windows 3.1
used: Windows 95

ahhh! old times...

3.1 betas in DOS-box
But actually installed, Windows 98

The oldest MS OS that I own is Windows NT 3.51.

I think my oldest OS I used was Windows 98, on my Nan's computer I can always remember that boot sound. Also my old desktop, I can't remember what OS it was but I'm sure it was one of the Pre-XP ones.

Windows 95 on a pentinum 3 128 ram and 4GB HDD. Good times. oh the joy of upgrading to windows 98. back then. XD

MS-DOS 6.2, and windows NT4 server.

My oldest OS is MS-DOS 3.3, with Windows 1.03. I still run em' sometimes on my IBM 5150.

My oldest MS OS was Windows XP Now Win7 and Win8 too

MS-DOS 3.3 on an old AT-class machine back in elementary school.

I use ms-dos 3.1, spanish version, in my first IBM PC Clone, (no my first computer). I have yet the disks and manuals.

My first (new $999.00) PC was a Tandy 386 1000 RSX (11/10/1992).

It had Windows 3.1 w/ MS DOS 5.0 preinstalled... thats the oldest I've used.

Windows 98 Second Edition

I used a Tandy 100 for research and I had a Timex Sinclair, but I am not sure that the Tandy used a MS OS. My Tandy 1000 ran under DOS 3. My oldest computer was a kit I got for Christmas when I was an undergraduate, in 1967 I think. It worked by plugging bits of wire into slots.

lol was windows 7 a joke xD it is more sold than windows 95 and 98 and xp

oldest os on pc, Windows 95 Swedish Edition,

Oldest on wmware windows 1.1 or something

thats mutch xD even windows 1.0 with new pc costed 99 dollar xD

Amstrad 6128+ with Locomotive BASIC OS. I think it was £399 when first released in 1985 but I got it in 1992 or 1993. learned a lot of coding using BASIC. Then it was Win 95 on a huge block of a Packard Bell desktop PC that had a Pentium MMX 9the tower was large). I liked the Packard bell wallpapers it came with (packard bell face)

my first computer was from Tandy, it was a cassette load and you know you had mastered it when the old Tennis game ran, oh how technology has changed, I do miss the programming fun.....not

real machine and VM : Windows 3.11 for Workgroups. Intel 486DX4 100, anyone? =D

I had win 3.1 on PSP (98 too) and 95/98 on a PC..

Owned: IBM OS/2 Warp 3 (Connect)
Used: CP/M-86

oldest I have a copy of is system 6.