The One Time Programmable (OTP) ROM is a collection of electrical fuses (eFuses) on the primary (14nm SOIFinFet) POWER9 die. It contains the very first instructions executed at power on, and is the ultimate root software executed by the first processor (the SBE) to come online during bootup.
The source code for the program fused in during manufacturing by IBM can be found in otprom_init.S.
The corresponding electrical fuses blown in order to encode this program are in OtpromFuseAlter.log, which is a manufacturing instruction file indicating which fuses should be blown using a laser during the assembly-test-packaging part of the manufacturing process. Each line specifies one fuse to be written. The pathname after the "ALTER" keyword on each line typically represents the hierarchical path from the root of the chip netlist (SPICE?) to the fuse cell to be blown. Reviewing the git history for these two files shows how this program has evolved over the various revisions of the silicon die.