How to autofill any formula till last empty cell

Like this perhaps.

Sheets("rxtcp").Columns("A:A").Copy Sheets("op").Range("C1")
LastRow = Sheets("op").Range("C"&Rows.Count).End(xlUp).Row
With Sheets("op").Range("D1").Resize(LastRow)
      .Formula = "=IF(ISNUMBER(SEARCH(""RXOTG-"",RC[-1])),RC[-1],"""")"
      .Value = .Value
End With