Option key works if the flash drive has an operating system the iMac knows.
This tip will help you determine what systems the Mac knows*
http://www.macmaps.com/docs/DOC-13695/
You can try booting into target disk mode with the 'T' key. If that works, and you have a Firewire compatible Mac, you can rescue data off of it. Thunderbolt to Firewire adapters exist from Apple as well. If Target disk mode does not see the drive in question from a known newer Mac, then the drive inside the computer likely is toast.
All startup combinations must be done from a WIRED keyboard. No wireless keyboard will work. Microsoft keyboards use the Alt key to represent the Option key and the Windows key to represent the Command key.
Note: Macs will not boot systems that are older than what shipped with them. Macs won't boot system specific discs that are from models and vintages other than the model in question (i.e. you can't use an early 2009 iMac installer disc on an early 2008 iMac).
*Links to my pages may give me compensation