Im going to assume youre using ArcMap for its relational database power rather than for spatial analysis. Or maybe youre prepping data for spatial analysis through a series of joins to avoid using Excel. When you import a dataset into ArcMap it will automatically generate a column called OID. It starts with then goes up. Ex 1 2 3 4. Open your file in Excel then Sort your data Ascending by the first column in your dataset Fill your new A column with a series 1 2 3 4 to the end of your dataset Open ArcMap then Import your Excel file Check OID and UID to make sure they match I hope this helps. Let me know if Ive missed something about your question. Feel free to private message me or send a screenshot.