Yes. Clean and degloss the oil based paint, apply an oil based primer, then apply the water based paint.
can i use water base paint over oil base paint
Yes, you can
No, you can't .
Yes, you can. You can always paint over with same paint.
That usually works quite well.
Not a good idea, better to use oil based primer.
Yes, you can.
Scrub it with warm water and a sponge sander block.
If the base coat used in your project is water base then yes, you bet you can. If there is any chance that the base coat used was an oil base paint then you cannot paint directly over it. The acrylic enamel will not adhere to the oil base paint and will begin to peel off as soon as it is dry. In which case a coat of primer is in order.
You can always paint latex ( water based )over an oil based paint as the latex just dries on the surface. However don't paint an oil based over a latex paint. Oil based paints need to absorb into the wood or other surfaces or it will just dry and flake off. For more information on the differences and when or not to use a latex or oil paint please see the following article. . http://www.ehow.com/how_4713170_between-latex-oil-base-paint.html
What they call "oil base paint" is really called "alkyd base paint." So yes, you can do that.
Of course can, we already proved it on offset printing..