I just finished watching Transformers Prime (haven't watch the movie yet) and...when Bumblebee killed Megatron my heart just dropped, and I think that Optimus's did too. When it just panned over to him with his wide optics as he watched the light go out of Megatron's...I saw a glimpse of Orion there.
And with all honesty, I don't believe that he will be able to live without him. Megatron was his mission, in the beginning the war, before it was even a war, Megatronus was there, they fought together, they were brothers. Then the betrayal came and it became about saving his friend and his planet. Eventually Optimus started to just seek revenge, for his brother and for his home. But at the end, I think that Optimus still had hope that he could save him. He wanted to save Megatron. And that chance was taken away from him.
And for the past 4 million years! Optimus' every waking moment has been consumed by Megatron. How would he live once the mission was over? Who would he be? I don't think he's ready to find the answer to that, and I don't think he'll survive to find out. I don't think he can.