Well, if he had warned Ola & Roth as soon as he learned of Mike's plans earlier that day, I would think that they would have been acting a little more carefully.

Wouldn't thay have had military or police guards immediately?

I doesn't make sense that if Fredo warned them earlier on, Ola was unprotected and the military guys waited until the last possible second to show up and save Roth.

So if Fredo did warn them, I think it would have had to have been at the party.

So, why did he wait until the last possible second, and again, who would he call?