My period was from September 25-28. I had intercourse with two guys. My ovulation days are from Oct 6-11. Guy a I had sex on September 25,28,29, October 2,3,6,7,9,10. Guy b I only did on Oct 28. Can guy b still be the father?

The odds may be a bit low but, only a DNA test will show paternity. Are you are trying to get pregnant?

How far along is your pregnancy? What method has been used to determine how far along you are-'last monthly period' or ultrasound?

How did you determine when your ovulation days were?

Without more information, yes, B could be. If you are usually regular with your periods, not having one in October could have just been an 'off' month that can happen for no reason. If you are using an app or other method that uses statistics to determine fertile days, it may not be accurate for your body. You may have been more fertile at the end of October than during the time you thought you were.

That said, DNA testing after the child is born will be the only truly accurate way to know.

Not impossible, but unlikely for guy B to be the father. When you have an ultrasound, you will have a better idea of how far along you most likely are in your pregnancy.

It's not impossible to be pregnant during the second intercourse (and with a different guy) but you can't be sure of that either. I guess, only a DNA test can determine that

This issue is, that first unless you were actually testing for ovulation, you do not really know when, online apps or phone apps, are not exact they are just normally for most people, and others can be exceptions to this.

So while guy b, is a lot less of a chance, it is possible, with the information given.
Estimated conception dates, after a ultrasound can give you a date that is normally good (plus or minus one week or so) at that point it may help narrow it down.