You just paint right over it, no special prep is necessary.
yes.
yes
You will need to use a sealant, such as Killz, to keep the stains from bleeding through the new paint.
can i use water base paint over oil base paint
Yes, you can, That's the normal way.
Yes. Clean and degloss the oil based paint, apply an oil based primer, then apply the water based paint.
yes
You can but it wont last, you will have a bigger peeling mess.
No, you can't .
That is not generally recommended.
Yes. Clean and degloss the oil based paint, apply an oil based primer, then apply the water based paint.
Yes, you can paint oil paint over sound water based paint easily, just be sure to degloss and clean the surface prior to painting.