This'll do it:

Sub Mimi()
Dim targRange As Range

For Each sh In Worksheets
With sh.Range("a1:a100")
Set c = .Find("Operating Projects")
If Not c Is Nothing Then
firstAddress = c.Offset(1, 2).Address
Set c = .FindNext(c)
If Not c Is Nothing Then
lastAddress = c.Offset(-1, 2).Address
If firstAddress = lastAddress Then
Set targRange = sh.Range(firstAddress)
Else
Set targRange = sh.Range(firstAddress & ":" &
lastAddress)
End If
For Each cl In targRange
cl.Value = "O"
Next
End If
End If
End With
Next