One of the best in the field. :) Although I think they just have dev assurance. Not total high assurance. An early one with total was GEMSOS by Schell, one of INFOSEC's founders:
http://www.iwia.org/2005/Schell2005.PDF
Smartcard OS by Karger, another founder of INFOSEC:
https://www.acsac.org/2012/workshops/law/pdf/Lessons.pdf
A comparable one for dev assurance, but maybe easier to emulate, was CompCert. Testing of it against many other compilers validated formal verification gave best reliability.
http://gallium.inria.fr/~xleroy/talks/compcert-lctes08.pdf
Ironically, Microsoft compete neck-to-neck with seL4 in terms of verification with their excellent work on VerveOS:
https://people.csail.mit.edu/jeanyang/papers/pldi117-yang.pd...