Update: I found Oreo!!! She is safely back home now. Thank you everyone for your help!

Update: The first two pictures are from another post where a redditor spotted her.

Her name is Oreo. She may respond when called by name. She may be scared outdoors.
Last seen: 6:55 PM, June 5
Location: Westmost corner of UW Columbia Lake Village residence (Waterloo)

She may still be nearby, possibly hiding or moving between backyards.

If you hear a cockatiel nearby, you can try a gentle flock call or whistle — she may respond to familiar sounds.

Any sighting, even small updates, would really help. Thank you so much for keeping an eye out.