Holding down the option key isn't allowing me to select Windows/OS X on startup
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