The schematic illustrates the positioning of the electrodes in a typical horizontal tube type corona discharge ozone generators:
As illustrated in the drawing, the gas flow between the glass/ceramic dielectric and the stainless steel ground electrode. In this case, the drawing illustrates oxygen flow, but in many systems air is employed. corona or plasma is formed in this space creating the ozone. Cooling water flows on the opposite side of the ground electrode.
The next photo shows a multi tube arrangement based on the design shown in the schematic: