Cat 3406 B diesel on oil, fuel injectors check, what else can be?

My truck have diesel on oil, we allready change fuel injectors o-rings, what else could be? thank you|||Could be a bad seal where the lines go through the head and onto the injector. Double check and make sure nobody left a line loose.|||well i%26#039;m not a diesel mech. but how about lift pump or cracked fuel port or hole in piston. does it smoke any. do compression test and test fuel port by air pressure. i liked to know too.