Boot Camp: is it possible to restart directly to Windows from Mac OS X?

The fastest way is to use Bootchamp. It adds an option in the Mac menu bar where you can just click and choose "Restart in Windows". Q.E.D.

The simple way is to use the Startup Disk preference pane in System Preferences. If you want to make it easier to get to you can find the preference pane at /System/Library/PreferencePanes/StartupDisk.prefPane. Note that this method won't work if you install NTFS-3G.

"Mavericks

sudo bless -mount ""/Volumes/BOOTCAMP"" -legacy -setBoot -nextonly; sudo shutdown -r now

Yosemite

Issue the command: diskutil list

/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *500.1 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 420.2 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
4: Microsoft Basic Data BOOTCAMP 79.0 GB disk0s4 <--That disk
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk1
1: EFI EFI 209.7 MB disk1s1
2: Apple_HFS 1TB 999.9 GB disk1s2

Then:

hdiutil unmount /Volumes/BOOTCAMP -quiet
sudo bless -device /dev/disk0s4 -legacy -setBoot -nextonly
sudo shutdown -r now

Important make sure that BOOTCAMP and disk0s4 are your actual windows partition. Sudo commands are dangerous, I take no responsibility if you use any of this commands. It works for me though."