The adding of oxygen into atmosphere of inert shielding gas (usually, carbon dioxide СО2) is available and helpful for the welding of steels. This allow us to stabilize electrical  arc and confluence of metal of welding bath with basic metal. 

Hi

by Adding a small amount of oxygen in the shielding gas, melt drops will be more relaxed and axial and As a result, there will be a smooth weld pool, and smooth weld pool, better wetting and finally flatter and smoother weld surface is achieved. It is necessary due to the amount of supplemental oxygen, the oxygen scavenger in the electrode used, that weld metal does not faced with metallurgical problems. Adding a small amount of oxygen in argon arc welding increases temperature also it cause more melt fluidity and stabilizes the arc. Oxygen reduces the surface tension and produces good penetration and fusion.in TIG process increased very little oxygen (less than 1%) will help to strengthen the arc.

attached article could help you..

good luck!

The limit content of oxygen about 30% (in mix with СО2).

In addition, Oxygen promote preferred nucleation sites (inclusions) to form articular ferrite, which is type of microstructure that have high strength and toughness at the same time 

Appropriate amout of oxygen in weld benefits the formation of inclusions, which promotes the nucleation of acicular ferrite, named oxide metallurgy or inclusion engineering. Comparison the oxygen in base metal of steel, about 20~50 ppm, the oxygen content in weld is often up to several hundreds ppm. If there were any detrimental effects on  weld strengeh and toughness?

Oxygen can be used by mixing it in case of inert shielding gas to form a weld bead from wide and shallow one to a deep and narrow one

You can have more information about how adding Oxygen can change the weld pool metal flow from outward to inward direction by reading this paper

((https://www.researchgate.net/profile/Hidetoshi_Fujii2/publication/43169651_Effect_of_Oxygen_Content_in_He-O2_Shielding_Gas_on_Weld_Shape_in_Ultra-deep_Penetration_TIG_Welding/links/5cc04bae92851c8d2200efe2/Effect-of-Oxygen-Content-in-He-O2-Shielding-Gas-on-Weld-Shape-in-Ultra-deep-Penetration-TIG-Welding.pdf