Hmmmmm. Okay, kids, let's back up a bit. First, DOS just means Disk Operating System, and it was around long before personal computers made their very first appearance. Big companies often ran under a DOS.
There were many different DOS OS's, once small pc's first arrived, too. I used a few of them, some on a TRS Model 1 (Radio Shack sold them). Some very big companies bought these, for diverse reasons, but only a few were used by anyone other than a hobbyist.
As to MS... B Gates was nothing until IBM decided to put out its first personal (instead of mainframe, mini, or super) computers. IBM needed a DOS for their machine, and their machine was quite different from the others out there. So, B Gates got hold of CP/M from its inventor -- bought all rights to it, then turned around and LICENSED his tweak of it to IBM.
At this point, many corporations suddenly wanted IBM PCs (now that IBM was making these boxes, they were suddenly not just for hobbyists or cutting edge types any more). And DOS was always along for the ride.
Initially, the apps were terrible. There were no drivers -- we wrote our own printer drivers (or embedded escape codes in our text files to communicate with the printer). Even I, working for a marketing research group as an 'end user programmer,' wrote a program that wrote lines to drive a plotter according to what the marketer wanted to see.
Meanwhile, out in a lab owned by Xerox, some very clever engineers had built a GUI. Guess who heard about it and requested a tour of their lab? The engineers begged the company not to give it away, but the company execs said, 'What do engineers know?' and so the tour went forward. Then Mac, followed by Windows 95, incorporated everything those engineers had developed, into their own new systems.
MS apps were still terrible (I could cite chapter and verse, but let's keep this short), but corporation top managers voted for having things all the same instead of having them good. All the same usually meant whatever app MS had just bought.
And so on... Just a bit of actual history for y'all, instead of all this stuff floating around in this thread (contributed by someone who lived through it).