Drop-down lists make it easy to fill out information in a worksheet, and you can use them in Excel Online, if they were added to the worksheet in the Excel desktop application. Also known as data validation, drop-down lists can’t yet be created in Excel Online. Here’s how you can do it if you have the Excel desktop application:
Click Open in Excel and then follow the instructions in this topic: Create a drop-down list.
After you’re done adding drop-down lists, save the workbook and it’ll be available where you opened it in Excel Online.
To stay informed about updates to Excel Online features, visit the Microsoft Excel blog.