There is an orifice in the center of the nozzle where only oxygen flows. When the workpiece is heated enough, the oxygen ignites the metal of the workpiece which results in a cut. The oxygen blows slag away from the workpiece. A nozzle for gas oxy-fuel is usually recognized by the trigger that starts the oxygen flow.
The nozzle is the most important part of the gas cutting equipment. It can adjust the flame's shape and the speed of the gases. The speed affects the cut; if it is too low, the cut will be rugged. On the other hand, if it is too fast, gas will be wasted and the shape of the cut will be irregular.
Gas cutting is only on option when working with some materials, for example unalloyed to low alloyed steel is suitable for gas cutting. That is because the melting point rises with the percentage of carbon, so the melting temperature is almost the same as the temperature of flame. Then what happens is that the metal melts and mixes with slag, making it impossible for the oxygen to reach, and therefore to burn the rest of the workpiece.
It is possible to gas cut very rough dimensions. So, the size of the machine or work table limits usually limits what a workshop can handle.