She's understand more than you think. She is saying goodbye and whilst it's distressing for you, she needs to understand that he has gone and where he has gone. When she is ready for you to take him away, she will let you know.
I'm so sorry you're having to face this. If you think that it's best to get her a friend ASAP then don't feel guilty, be glad she had the love of Oscar and that she has the love of you. It's not replacing Oscar, it's just giving her a new friend, and when times are tough, we all need friends.