I believe that I was able to find a solution to the problem (although Cell's choice is rather strange, and I have not quite figured it out yet, for example, my formula uses A1, which actually means C1 because of the selected range).
Here is the code I used for someone else:
string condition = @"=OR(ERROR1, ERROR2, ERROR3)"; var cfOR = (FormatCondition)targetSheet.get_Range("C1", "C10").FormatConditions.Add(XlFormatConditionType.xlExpression, Type.Missing,condition), Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing); cfOR.Interior.Color = 0x000000FF; cfOR.Font.Bold = true; cfOR.Font.Color = 0x00FFFFFF;
Note that the FormatConditions.Add () method has a different signature for different versions of Excel interaction.
source share