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Depending on the welder design you might, in principle, be able to use it as the basis of a DC-to-AC inverter. Such a project is not for the faint-hearted. There are numerous threads about problems people experience when trying to make inverters.
The only oil filled welders I have seen have just been transformers with some means of stabilising the current. (Magnetic shunts or added inductance.) There is a remote possibility that an inverter welder could be converted to convert DC to AC but I think it would be more expensive than buying an inverter. The fact that you think you can use a transformer with DC indicates that your electrical knowledge is far too limited to even consider designing the modifications to an inverter welder.