Excel Online supports Conditional Formatting with Data Bars, Color Scales and Icon Sets. Select a range to be formatted, and go to Home > Conditional Formatting > Pick your style. If you need to clear an existing format you can choose the Clear Rules option.
If you want to apply other Conditional Formatting options, and have the Excel desktop application, you can use it to add and change conditional formatting. Here's how:
Click Edit in Excel and follow the steps in add or change conditional formatting.
After making changes and saving your workbook in Excel desktop application, you'll see the changes when you reopen it in Excel Online.