Hello 

You can apply for PSU'S as soon as the online forms available on the official website of that particular PSU.Generally many PSU's release the forms before gate result.In that case you just have to enter the Gate Roll no.If you qualify then they call you for the Interview.

Although many PSU's like ONGC release the forms after the result.So you can regularly check the site and fill the form as soon as they are available.

I Hope it help's

GoodLuck...