Ola arrives by boat. There is a reference by Michael to getting "Johnny's men something to eat", so we know he didn't come alone. But the dead guys couldn't have been Ola's men, 'cause they would have been recognized from earlier in the day. And presumably Ola and his men left the way they came. I don't think he stayed behind (besides, he looked a little old and soft to be going around killing hired killers, one against two), and even if it was him, how did he get away?
Seconds after the shots were fired, the whole compiund was on lockdown.

I guess years from now I'll still be shooting down these theories until I find one I can live with.

If I seem a litle crazed about this, it's only because I think the question is SO important because of the statement Michael makes to Tom, to the effect "Unless I"m mistaken, they're (the assasins) dead already... killed by someone inside". Who did Michael think that was? Or even if he had no idea at that point, assuming he was right, who was it? Again, surely not Fredo.

And this stone in my shoe has been bothering me for 30 years