The option buttons have a property called 'LinkedCell' When you type the cell reference in for 'LinkedCell' Excel will automatically change the value of the cell based on your selection. To get to 'LinkedCell' depends on whether you have used the 'Form' toolbar or the 'Controls Toolbox' toolbar to create you option buttons.
Note: I'm using Office 2003 I hope your settings will be as easy to find.
Forms:
If you have used the 'Forms' toolbar to make your option buttons (you probably had to create a group first):
Right click on one of the option buttons (this selects it). On the 'Forms' tool bar click on Properties. In the 'Format Control' window click on the 'Control' tab. In 'Cell link' type or navigate to the cell you want Excel to update with your options. Click 'OK' that cell will now update with an ID number for the option you have selected. 1 = the first option you created. 2 = the second you created etc... Therefore the 1,2,3 etc identify the selection.
Control Toolbox:
These option buttons don't appear to need to be in a group. For each option you create it has it's own properties and own LinkedCell. Make sure you have the 'Design Mode' turned on in the 'Controls Toolbox' and then right click on an option button. Click properties in the menu that opens. Find the 'LinkedCell' property and give it a cell reference. Do this for each option you have and use a different cell reference for each option. Turn design mode off. When you click on the different options their corresponding cell should say 'TRUE' or if unselected 'FALSE'.
Hope this helps.
regards,
Douglas