Holding down the option key isn't allowing me to select Windows/OS X on startup I have Windows 7 installed on a 15-inch Retina MacBook Pro.

According to Apple's Boot Camp Installation and Setup Guide, pg. 10...


  Select an operating system during startup
  
  You can select which operating system to use during startup by holding down the Option key. 
  
  This displays icons for all available startup disks and lets you override the default setting for the startup disk that is in Startup Disk preferences (OS X) or the Boot Camp control panel (Windows), without changing that setting.
  
  1 Restart your Mac and hold down the Option key until disk icons appear onscreen.
  
  2 Select the startup disk with the operating system you want to use, then click the arrow beneath the icon.


I'm finding that holding down the option key has no effect -- the machine boots into whichever OS was last running.

This is true whether I'm starting from the shut-down condition, or restarting.

I've tried hold down each of the two option keys and I've tried holding down both at once.

I've tried pressing the keys from before the startup chime and just after.

The only way I've been able to control which OS starts up is to restart from the Boot Camp control panel in OS X in either OS.

Any thoughts on how to get the machine to behave as described in the docs?
I had a similar issue and the below trick solved it. 


  Resetting NVRAM
  
  
  Shut down your Mac.
  Locate the following keys on the keyboard: Command (⌘), Option, P, and R. 
  Turn on your Mac.
  Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
  Hold these keys until the computer restarts and you hear the startup sound for a second time.
  Release the keys.
  


Source: https://support.apple.com/kb/ht204063