Probably some version of DOS, I can't remember. First Linux distro was Fedora Core 4.

MSX, some version of MS-DOS, DR-DOS, Windows 3.0, 3.11, Slackware 3.0, Windows 95,98,NT,XP,7, CentOS, and now, MacOS.

At work, Windows XP,7 and 10, and several versions and flavors of RHEL.

Commodore64 for first OS, taught myself to type and then taught myself BASIC on that beast.

I honestly do not know what my first Linus distro was, whatever was on the machines in the CS half of the computer lab in college. First one I installed myself was Ubuntu, but I abandoned it almost immediately in favor of another distro that I also don't remember.

Well, this sets a new level of recent here judging by the comments.

Desktop OS: Linux Mint 20 MATE. Yep, that's right. I only got my first proper computer in 2020.
Thankfully, I had to install the OS myself, which was of course preceded by choosing an OS.
I had Windows 10 on that laptop for 2 days which served me to compare different OSs and burn the install DVD. I had no flash drives, and just dug out one old DVD-RW. OK, I'll be honest, hearing about Linux first I was searching for "just Linux", pure Linux, not derivatives. Oh well, GNU+Linux copypasta actually being helpful.

Alright, but why did I "have" to install an OS if I got it with Windows? It was used. I did reset it, but even though it was my first proper PC, I had no lack of paranoia. I thought that someone before me could have put spyware on that.
And I was right. Not the way I thought, but I was. That someone was Microsoft.

First OS: MS-DOS 5

First Linux (many years later): Yellow Dog on one of those dome-shaped iMacs with the PPC chips

First OS - DOS 5.0 First Linux - Knoppix

My first OS was DOS 5.0, I switched to linux with Fedora core 6

I had a Debian PC for a while, it was more of a project. IT work still revolved around Windows back then. Later I discovered Puppy Linux and ran that on a live USB. I encrypted the hard drive in my work laptop and never had to worry about anything making from the USB to my work data. Not a bad way to live, only had to carry around one laptop when I traveled for work.

It was a Lisa. Those were good times.

First linux distro was Ubuntu but I used it in VM but the actual daily driver was Archlinux. I think Windows XP was the first OS which i used on computer(In home sure but in school maybe it was Windows 97?)

My first OS was Windows 98.

Linux distro was Ubuntu 6.0.

Atari 800 basic.

Gentoo was the first distro I used for any significant amount of time (college).

Windows → Mint → Windows → Void → NixOS

Macintosh System 7. Then I moved to Windows 98, which was the style at the time.

CP/M on a Kaypro II. My uncle was a contractor for the US air force. Even had a modem - a wooden box he built to hold a telephone handset.

Fun times.

First OS for me would be Windows XP.

First Linux I've tried using could technically be whatever version of Android was on my mom's Droid phone.

First desktop Linux would be some version of Ubuntu around 2014-2015

The first one I remember using was good old Windows 98.

School: if it wasn’t a Macintosh Plus it was something like it. We were given very little time with it and had to go to a special computer room to use them so I don’t remember a lot of specifics about it, beyond the school tech guy having to painstakingly load each program we wanted to use manually from a floppy

Home: Windows 3.1. I don’t think we got the internet until Windows 95, though

First operating system was probably either Windows 98 or Windows 2000. First Linux operating system was Ubuntu 10.10.

Slackware 1.0, dann S.u.S.E

Zx spectrum --> msx-dos --> ms-dos --dr-dos --> win95 --> Suse (before it was bought by novell) --> win98 --> debian

Windows 7 -> Windows 10 -> Kubuntu -> Linux Mint -> Nobara -> Pop!_OS -> OpenSUSE TumbleWeed -> EndeavourOS

Reading the comments I want to say I feel old (25y) but compared to others I feel like a gen alpha

I came late in the tech world. I started with Windows XP

And my first distro was Ubuntu 10.10 till Ubuntu 17.10. Now I just hop from distro to distro. Right now using Pop!_OS for a year now.

First OS: Windows ME (shudders)

First Linux distro: Some random one designed to run on Netbooks (Couldn't get wireless to work if I remember correctly)

Windows XP on a laptop. Then Windows 7 on a new laptop. After that, Windows 10 and Windows 11 on desktop and another new laptop.

Tried Debian on my laptop. Later, switched completely to Linux Mint on desktop. Distro-hopped to Kubuntu (KDE Plasma). Wanted to get Plasma 6 immediately after release, so I installed EndeavourOS on my desktop and laptop.

Now switched to pure Arch Linux on my desktop PC, didn't boot Windows on any of my private PCs for months (no dual boot, only GPU passthrough VM).

First os: technically the Wii's os and android

First Linux: Ubuntu 16.04 (I remember it like it was yesterday 🥲)

Windows 3.1, 98SE, XP, 7, 8.1, 10 and now 11.

I did once use Ubuntu for a few weeks but this was after someone at school goaded me into acquiring a copy of Norton PartitionMagic to try and merge two awkwardly partitioned drives on my computer, which then nuked my C:\ partition. I didn't have a backup copy of Windows XP to reinstall from so I had to go open source.

Needless to say, we didn't remain friends after that.

Apple ProDOS yo! Then System 6 and on.

First Linux was a Debian disto packaged with Linux for Dummies somewhere in the late 90s/early 00s.

My first OS was whatever ran on a Commodore 64. I guess the Commodore kernel and Basic?

My first distro was whatever version of Fedora was current in the fall of 2008. I'd gone to university that year and my laptop crapped out. Couldn't afford a legit Windows license at the time to replace it, and I'm pretty sure I just remembered that Red Hat was a thing and found Fedora that way. One thumb drive and 16 years later, still using linux, so I guess that was about the only good thing to come from my abortive first attempt at higher education.

Personal computer? Windows 3, I believe, but worked on computer earlier than that by a couple of years - I'm not sure what OS, it was maintained by this old guy named, no kidding, Mr. Fox. Had a rudimentary spreadsheet program.

DR-DOS

Mac OS 7. By the mid-2000s, I adopted Arch Linux after looking into some other Linux distributions.

On my first PC it was DOS 2.0, but before that I had a Vic-20, then a Commodore 64.

Apple DOS Edit: Apple ProDOS
First Linux, I'm not sure anymore. Whatever was prevalent in the mid 2000s.

Windows XP, but it was during its ending phase, so I think it really was Vista. My first Linux distro was Kali Linux because I wanted to be a cool hacker when I was a kid. I never got too much into it then, though. I then found Ubuntu, and strangely enough, I switched to Trisquel, which wasn't too bad. I decided to go all the way and buy a T400 with Libreboot/Trisquel when I was about 15 years old and used that as my second computer for about two years. I learned how to start installing Libreboot myself. It was a really fun experience (not really, there was a lot of quitting and crying), but it taught me more about GNU and the entire philosophy. I started to learn more about GNU and RMS when I was 18. Now I'm 20 and use Arch. The end.

First OS on a computer I personally owned? Windows 98. First Linux distro was Source Mage.

If not counting ownership, then Apple IIs at school and then slightly later my family got an Amstrad that was primarily a DOS machine, but could also boot (by switching floppies several times) to some sort of GUI.

DOS - Win3.1 - Win95/98 - BeOS - Red Hat Linux - WinXP - Mac OS X - WinVista/7/8 - MacOS X - Win10 - Debian Linux (and staying with it).

On the Linux side: Crunchbang Linux when it was running Debian

I tried Microsoft Windows & Apple OS X before that, but that is is now behind me… as is Debian & Arch being a NixOS user for the last few years.

First OS: Win 95 (As a wee lad). First Linux: early lubuntu and backtrack that I used in college around 08-09.

Windows 98. Later the first Linux distro I tried was Mandrake Linux but I didn't really get into it until later with Crunchbang.

Microsoft Extended BASIC