It wasn't until Episode 38 (Over half way through the series!) titled "Up For Air" that we learned the power of the fourth crystal.
In Episode 33: Captured, we realize that Quellor has stolen the plans to Gimmick's Airship and constructed his own. He uses it to chase the trio and the Anythings with them until they give up and are captured as the title purposes.
Teddy Ruxpin had hidden the Crystals in the "Eclipse" (the M.A.V.O. Airship) which later crashed at the bottom of Leakee Lake. He later goes down in search of them and brings them back to Gimmick's house.
While Quellor's men try to pull up the ship themselves in search of Quellor's Black Box, Teddy goes down under in a diving suit he found.
In Episode , Dredge, cousin to Drudge of M.A.V.O., is ordered by Quellor to go down in search of the Black Box. When Dredge (who is an octopus-like creature) finds the "Eclipse", he also finds Teddy who is in search of the Black Box as well. He runs from Dredge who knocks Teddy's helmet off, preventing him from breathing underwater.
Teddy holds his breath until he is trapped in the cave, the Crystal Belt around his waist. After some time, Teddy realizes he is able to breath somehow and uses that to get to the surface. He pops his head out above the water and realizes it was the fourth crystal which had given off the oxygen supply he just used.
This is the only time the crystal is used. There is no other need of it throughout the series. The only other time the crystal is mentioned is in Episode 63: On the Beaches, when Gimmick decides to demonstrate each of the crystals powers and in Episode 65: The Mystery Unravels, when Teddy reads from the Crystal Book.
Other Places to Find The Oxygen Crystal
Episode 5: Escape From the Treacherous Mountain