Posted by Robert Hernandez on Sep 11th 2026
| Term | Definition |
|---|---|
| O.D. | Outside diameter, the exterior width of a pipe, tube, or any round object, measured at its widest point. |
| Off time | In resistance welding, the time between repeat cycles, when the electrodes are off the work. |
| Ohmeter | An instrument for measuring the resistance to the flow of electricity through a circuit. |
| Ohms | The unit of measure for resistance to the flow of electricity through a circuit. |
| Open arc | An arc that is open, unaltered, or not shielded by a constricting nozzle. Also, a visible arc. |
| Open-Circuit Voltage (OCV) |
With an open circuit, no current flows. The voltage is impressed upon the circuit so that when the circuit is complete, current will flow immediately. OCV is the voltage in a welding machine circuit when it is on but the arc is not on. |
| Operator controls | In robotics, the controls used by an operator to start a programmed operation. |
| Organic | Originating in once-living matter; animal or plant, not mineral. |
| Organic fluxes | Fluxes that contain carbon. They are corrosive during the welding operation, but become non-corrosive when the welding is complete and can be washed away with water. |
| Orifice | The opening through which wire, gases, or a plasma arc passes. A precise hole that may be in a regulator, torch, or torch tip. |
| Orifice gas | The gas that surrounds the electrode in plasma arc welding and cutting. It becomes ionized in the arc to form the plasma. |
| Orthographic projection | A method of developing a mechanical drawing, based on viewing an object as if it were in a transparent cube. |
| Oscillating | Moving back and forth in an uninterrupted manner. |
| Oscilloscope | An instrument that shows electrical impulses on a calibrated screen. |
| OSHA | The Occupational Safety and Health Administration, the federal agency responsible for safety in the workplace. |
| Other side | The area of the welding symbol above the horizontal reference line. Also, the side of the welding joint opposite the side touched by the welding symbol arrow. |
| Outfit | Refers to all the welding equipment needed to make a weld. |
| Out of position weld | Welds made in the vertical, horizontal, or overhead positions. |
| Outside corner weld | Fusing two pieces of metal together, with the fusion taking place on the under part of the seam. |
| Overhead position | A weld made on the underside of the joint with the face of the weld in a horizontal plane. |
| Overheating | Applying excessive heat, and thus damaging the properties of the metal. When the original properties cannot be restored, the damage is referred to as burning. |
| Overlap | Extension of the weld face metal beyond the toe of the weld. |
| Oxidation | The process of oxygen combining with elements to form oxides. |
| Oxidation resistance | The ability of a material to resist the formation of oxides. Metal oxides occur when oxygen is combined with metal. |
| Oxide | A chemical compound resulting from a combination of oxygen with other elements. |
| Oxygen | A gas formed of the element oxygen. When it very actively supports combustion it is called burning; when it slowly combines with a substance it is called oxidation, as in rusting of iron. |
| Oxygen arc cutting under water | A process that uses the arc from a hollow electrode to heat the metal, and pressurized oxygen flowing through the electrode to make the cut. |
