how to quickly reboot from OSX to Windows and back

Note: This does not work on systems with System Integrity Protection enabled

Write a shell script that does this

#!/bin/sh
# tell OSX to change the boot disk 
# nextonly just for the next time - without the default would be Windows
/usr/sbin/bless -mount /Volumes/BOOTCAMP --setBoot  --nextonly  
# reboot
/sbin/shutdown -r now

This can be called from a terminal window

sudo bootcamp

The use Automator to call this via an apple script and save as an application

on run {input, parameters}

    do shell script "/Users/mark/bin/bootcamp" with administrator privileges

    return input
end run

Another way to get from OS X to Windows quickly is to use Bootchamp, which adds a menu bar item like this:

For my Windows-booting needs Bootchamp has worked well.

(I learned about it from a similar question on Super User I once asked.)

Hold ⌥ while booting, you'll get a prompt to choose which OS / drive you want to boot from!

Here is an Automator workflow I wrote to accomplish rebooting to Bootcamp automatically:

https://github.com/ryanmcginnis/Restart-to-Bootcamp

As of El Capitan, it is no longer possible to programmatically set the startup disk, even with SIP disabled.

It seems this script from SuperUser handles the closing of open windows (and stopping the scheduled restart if user presses 'cancel'):

do shell script "hdiutil unmount /Volumes/<Windows_Partition> -quiet"
do shell script "bless -device /dev/disk0s3 -legacy -setBoot -nextonly" with administrator privileges
tell application "Finder" to restart