The May 1975 issue of IEEE Computer magazine had a one-page ad from MITS for the Altair 8800. In the ad, they mention their "DOS Extended BASIC Language System for $6649" for an Altair with 16K memory, terminal and interface, "disc controller and 2 disc drives, DOS and Extended BASIC software."
An IMSAI ad in June 1976, mentions a "floppy controller with on-board processor and DOS"
So obviously not only was the term DOS being used for Micro-Soft's BASIC-oriented OS in 1976, it was being used on 8080 computers from multiple manufacturers, and as we established earlier, being widely pirated (or at least widely enough for Bill to complain loudly). Case closed, and be careful arguing with old guys about history they lived through. Sometimes we can still remember stuff!
"To IBM, 'open' means there is a modicum of interoperability among some of their equipment." -- Harv Masterson