Poor Oscar.  Gilly, when I recently got my second maltese x rescue dog (9yo), I aleady had a male 4yo Mx.  I was concerned that he would fret and feel threatened by losing his place in the pack by the new one. 

I always made sure I fed him first, patted him first, led him through a door first, gave him extra cuddles etc. .  This is dog language and reassured him, and also gave the message to the new one that he was ahead of her in the pack.

Geez, what a convoluted post. Hope it makes sense Gilly.