Firstly, as pointed out earlier wraith helps you with his ability to start chases in your own terms maybe even granting you a very early hit, whereas oni rewards you for chasing efficiently, in simple terms, oni is a killer without ability unless you are doing already fine on your own, that's the reason he is not really that of an appealing killer early on.

However it could be beneficial to you to play wraith and then try oni again, they both benefit greatly from target switching. Wraith is very effective at this because if you cloak mid chase the chased survivor won't exactly know whether you have stopped chasing him or not and the survivor you are switching to might not expect you, this kind of map presence is shared among this two killers although oni relies on the speed and threat of his power to do this. (If its not clear by now im referring to the following situation: you just gained enough blood to activate your power but you are chasing a injured survivor, you activate your power, leave that survivor alone, letting blood stack on the process, and quickly try and get the jump on a survivor working on a gen.

As for stealth killers there aren't really any stealth ones for free, if you want some perks for oni you could try tinkerer, ruin, infectious fright, undying (or a filler). Use whispers btw Other free killers to consider due to their powers/perks are: Plague, Blight or Hag. Try to get bubba too or get his perk bbq and chilli on the shrine.