A wood column have been coated with water based polyurethane over the natural surface and I would like to stain it with oil based wood stain. How can I do so?
If you want to use a conventional oil stain then this won't work. Conventional stains of all types require direct contact with the wood surface, so if there's a full finish in place it needs to be removed. If you try to use oil stain over a clear finish at minimum you'll get very light colouring, at worst you'll get virtually none as when you go to wipe away the excess (always done with oil-based stains) it will all come off.
There is an alternative and that's to use "gel stain". Despite the misleading name this is not stain, it is a coloured varnish that has been thickened to a light jelly consistency. If applied over another finish the surface must be clean and matt, so after any cleaning that's necessary you need to lightly sand, or scuff the surface with an abrasive like steel wool or a synthetic equivalent. After this you apply the gel stain as directed. Multiple coats can be applied to build up the required depth of colour as long as sufficient drying time is left between coats.
You will also need to apply a final finish, as although "gel stain" is based on varnish it's too easily damaged by itself and needs the additional protection of a clear finish on top for durability.