Conor's really only shown major weakness standing in the Nate fights and the Poirier rematch. He's not fought much at LW, but just based on the Alvarez and Cowboy fights (common sense) his FW striking transitioned well to LW. Most stand-up fights he's involved in are blowouts. But current Conor could be declined, maybe doesn't get back to that level or won't improve, so depends on how he does in his next few.
Poirier's faced way more top LW's and beat him last time, but overall against really everyone he's been more vulnerable to getting outstruck, dropped or KO'd. Wars. He's never really outclassed anyone. It's a grind.
Current Oliveira is difficult to beat standing, but has defensive weakness, shots get through his guard and he doesn't move his head, takes the full impact. Yet outside of getting dropped by Chandler he's been dominant on the feet for a while.
Don't get me wrong, Poirier is up there, he is very good, but if you look at the damage he takes it doesn't scream the best to me.