If I create or purchase an oil with a harmful spell like say Magic Missile. Can I throw it as a splash weapon?
Oils are for smearing not throwing. An offensive oil would have to be smeared on an unconscious creature. (And most characters won't want the weird reputation that comes from smearing oil onto their downed foes.) The rules don't even consider trying to smear an oil on a conscious, unwilling creature, nor do the rules cover trying to pour a potion down an conscious, unwilling creature's throat.
A reasonable house rule would allow a creature to subject a grappled or pinned victim to oil-smearing or potion-pouring as a full-round action that provokes attacks of opportunity (not usually from the grappled or pinned victim, obviously).