there are also on0_<?php echo $WA_Store_Cart_Index+1; ?> / os0_<?php echo $WA_Store_Cart_Index+1; ?> elements on the page that can be used for bpassing option information.
<input type="hidden" name="on0_<?php echo $WA_Store_Cart_Index+1; ?>" id="on0_<?php echo $WA_Store_Cart_Index+1; ?>" value="Option Name" />
<input type="hidden" name="os0_<?php echo $WA_Store_Cart_Index+1; ?>" id="os0_<?php echo $WA_Store_Cart_Index+1; ?>" value="Option value" />
the on0_<?php echo $WA_Store_Cart_Index+1; ?> element is used for setting ooption name (i.e. Color or Size)
The os0_<?php echo $WA_Store_Cart_Index+1; ?> element is used for passing the value (i.e. Red or Small)