As TTT says ^
painting over the now chipping paint will be a dire mistake. Once you apply wet oil based paint to chipping emulsion, the weight can cause it to flake off in bigger sections, bringing the wet oil paint with it. God help you if that lands on a carpet ;)
If this doesn't happen, the best you can imagine will be flakes coming off the wall as you paint, almost like when you start re-painting a half dryed wall. It will cause flakes and speckles of dry paint in the new wet coat.
It will be messy.
Maybe get a painter in just to do the prep work, then paint yourself to save money??