Some companies do announce layoffs to their employees ahead of time, but I suspect that most companies don't. All companies I have been at when there was a layoff, there was no warning given.
I suspect it depends upon the relationship between the company and its employees. I know one guy that was laid off and stole a hard-drive on his way out of the building. I am not sure what damage that guy would have done if he'd had a month to sabotage something.
There are a lot of seriously nasty people working at Oracle. I think you should be glad that Oracle doesn't give a long period of notice. We are all safer that way.