How do I replace NA values in Column B with opposite value of column A

I have a large dataset. Two of the columns contains YES and No Values. Column.A has 0 NA values while Column.B contains around 40% NA values and dataset holds 500 lines where Col.B is the opposite of Col.A

Sample Columns

Col.A          Col.B
Yes             No
Yes             NA
No              Yes
No              yes
Yes             No
No              NA

I'm wanting to replace all the NA values in Col.B with what ever the opposite is of Col.A

I have been trying to replace all of Col.B based on Col.A Value using a for loop

for (j in 1:500)
{
  if (Col.A == YES )
  {
    Col.B = "NO "
  }
  else
  {
    Col.B = "YES "
  }
}

However the IF statement is coming back with 'YES' not found