An open libultra decompilation brought you to by a couple of clever folks.
This is the N64 standard SDK library. It is normally closed source, but recently has been decompiled (with a 1:1 match when recompiled using the original compiler) from the game Super Mario 64. Unfortunately, there are a couple of functions that I have deemed useful that were not used in SM64 (and were discarded by the linker) and are thus included from the libultra source code provided from the N64 SDK. The Oman archives were not used in any way for this project.
Building libreultra is exclusive to Linux and WSL due to the IRIX emulator. Before you build, install the following dependencies (Debian package names):
makegitbinutils-mips-linux-gnu(>= 2.27) (mips64-elfis also acceptable)python3build-essentialpkg-configzlib1g-devlibglib2.0-devlibpixman-1-devlibcapstone3
After installing these dependencies, run make in the root directory.