Why is there a hole in the AC reservoir compartment?

The hole or slit is an air-release vent to pre­vent over-fill­ing of the ‑O2 compartment. 

When the com­part­ment fills, the slit opens and releas­es excess.  It pre­vents rup­ture of the com­part­ment and dilu­tion of oxy­gen-rich +O2 air.