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

Sub ScratchMacro()
'A basic Word Macro coded by Gregory K. Maxey
Dim oRng As Range
  Set oRng = ActiveDocument.Range
  With oRng.Find
    .Text = "[0-9Oo]{1,}"
    .MatchWildcards = True
    While .Execute
      oRng.Text = Replace(oRng.Text, "O", "0")
    Wend
  End With
lbl_Exit:
  Exit Sub
End Sub

Sub O_To_Zero()
''Replace letters "O" followed and/or preceded by a digit
'with zeros.

    Set aRng = ActiveDocument.StoryRanges(wdMainTextStory)
    With aRng.Find
      .ClearFormatting
      .Replacement.ClearFormatting
      .text = "O([0-9])"
      .Replacement.text = "0\1"
      .Wrap = wdFindStop
      .Format = False
      .MatchWildcards = True
      .Execute Replace:=wdReplaceAll
      .text = "([0-9])O"
      .Replacement.text = "\10"
      .Execute Replace:=wdReplaceAll
    End With
End Sub