I'm so, so sorry. I know Oreo was greatly loved.
I've lost a beloved dog (Leo) suddenly and with no explanation, and a beloved dog (Murphy) within a few months of adoption. I won't pretend I know what you're going through, because we all feel our own grief in our own ways, but I can say, it will feel horrible, absolutely horrible for a while, but eventually (and I know it doesn't feel that way right now), the pain will fade, you'll remember the good parts, and it will get easier.
Godspeed Oreo and please send a message to your family, I know they love you very much.