Go to the oracle is the name given to five missions in Jak II. Jak could visit the oracle in the Water Slums at its hut, after the mission "Find pumping station valve". The oracle sensed a "dark rage" burning within Jak, seemly being aware of his new Dark Jak alter-ego. He offered to help him gain control over his powers, but requested skull gems as a sign of slain metal heads, the enemy of the Precursors. In total, the oracle gave Jak control over four powers, all while talking of cryptic messages related to young Jak, Metal Kor and the time travel behind the plot of the series.
"Go to the oracle" missions are among the shortest in the game, in itself requiring no more than entering the hut and viewing a cutscene. The real mission lies in collecting all skull gems, of which there is a fixed amount until after the game is completed. There are approximately 550 gems, but the oracle requires 525, and it is highly unlikely you will be able to collect said amount without the gems found at the Metal Head nest. Nonetheless, all metal heads drop skull gems after the final mission, meaning it can still be completed afterwards.
Due to this, these missions are not technically required to complete the game. When you have the required number of gems for the next power, the oracle icon automatically appears on the minimap, indicating you can visit to learn a new power. Afterwards, an on-screen instruction appears as to how to use it. The first time the oracle simply instructs you to use to summon Dark Jak.
The first power obtained is Dark Bomb and requires only 25 skull gems. The second power is dark blast, requiring a total of 200. The third is Invincibility and again requires 200 gems. The final power is Dark Giant, which requires only 100 skull gems.