Paul beat me to it.. HP's SPL and Tandem's TAL were old news by then ... In fact the HP's 3000's and HP 9000's stack architecture was model from the B5000, see below...
> On Dec 4, 2018, Greg 'groggy' Lehey <grog@lemis.com> wrote:
>
> The original Tandem OS (called Guardian at the time) was written in Tandem's TAL (Transaction Application Language, amongst other productions), a vague evolution of HP's SPL that looked more like Algol, starting in about 1974. That is also the earliest I know of an operating system being implemented entirely in a high level language.
Most likely the earliest operating system written in a high-level language was the one for the Burroughs B5000 (early 1960s), written in a dialect of Algol 60.
Called Executive Systems Problem Oriented Language or ESPOL. A later edition (66/67 time frame) of the reference manual for the 5500 can be found on bitsavers as: ESPOL B5500 Reference Manual 1967