OAW is an obsolete process that is almost never used. It produces welds of inferior quality. Professionals use arc welding methods for just about everything.
Advantages of OAW - doesn't require a source of electricity so it's very portable.(however gas engine driven welder-generators for arc welding are standard equipment these days.)
Disadvantages- gases are expensive, extremely flammable, and dangerous. Very slow production rate, excessive heat input causing warping of material and large heat-affected zone, produces welds of low quality. Equipment is difficult to use. Not practical for steels > 3/16" thickness. Can only used for steel or copper alloys.
Advantages of arc welding methods- produces welds that are stronger, much faster, much lower heat input, and have better quality than OAW in every sense. Can weld metals of any thickness, (TIG/GTAW is useful for very thin sheet metal) Can weld most metals, steel, stainless, nickel, copper, aluminum, etc.
Disadvantages- requires a source of electricity or a purpose-built generator. Produces intense light, radiant heat, and ultraviolet rays making special protective equipment necessary. Typically produces lots of sparks and spatter which can cause a fire hazard. Electric current can be dangerous if standing water is present.