Sub LetterToZero()
'Replace with zero any digit string of any length containing the letter O
'Test sequence: This sequence 145O or this 4O56 or this O4578
    Set aRng = ActiveDocument.StoryRanges(wdMainTextStory)
    With aRng.Find
      .ClearFormatting
      .Text = "O[0-9]"
'      .Text = "[0-9]O"
'       find letter O inside digit string
'      .Replacement.Text = "0" 'Replace only the letter O
      .Wrap = wdFindStop
      .Format = False
      .MatchCase = False
      .MatchWildcards = True
      .Execute
'If .Found = True Then MsgBox "Found " & aRng.Text
    End With
End Sub