PL/I, PL/S variants, macro assembly (eg HLASM), assembly, and microcode that I can tell. PL/S was an interesting language that was high-level, not GC'd, and gave programmers control over how each function compiled. Pretty powerful for 1968. See BSL specification PDF in references section.

https://en.wikipedia.org/wiki/IBM_PL/S

Oh true forgot about mainframe things. It also looks like VMS has other languages than C: https://en.wikipedia.org/wiki/OpenVMS

I don't really know where the major/minor cutoff is myself. :)

Pretty much every OS I can think of is in C - Windows, *nix, RTEMS, INTEGRITY, VxWorks

Remember that a few of those RTOS's, although OS itself is C, have runtimes for Ada and Java that apparently sold well enough they continue to be updated. Only RTOS I know still done in Ada is MaRTE OS:

http://marte.unican.es/

Well, Muen is done in SPARK but it's a separation kernel rather than full OS:

https://muen.sk/#_features

That it was integrated into GenodeOS framework shows one could build a system around it but it's not quite one itself. There's also the JavaCard OS's for low-level stuff and even Java CPU's on market.

TemplOS is in HolyC. Plan9 uses a C-like language. Haiku is written in C++. Those are all minor OSes, but I've at least heard of them.

http://stackoverflow.com/questions/6398305/operating-system-... lists some OSes written in Ada.