Olin Gisir meant "secret keeper" in Elven, which was precisely what these scholar-mages were. Elven lifespans were many times those of humans, and this brought along with it secrets that were best left secret.
Olin Gisiae were elite elf or half-elf lorekeepers who took it upon themselves to protect dark secrets and ancient magics from those who would have exploited them. Such duties made them much like the Cult of the Shattered Peak.
The more powerful these secret keepers became, the more skilled they became at spellcasting, being able to cast more spells per day. They also gained access to spells that they did not have access to before, such as dispel chaos, dispel evil, or dispel law. They could also cast dictum, holy word, or word of chaos, as well as being able to cast the augury spell more often. Due to their diligence to keeping dark lore hidden, they became far more knowledgeable about both history and the arcane, and were more proficient at using magical devices.