How can I boot my dual-booted Mac back to OS X?
Hold ⌥ on boot and select your OS X partition. This changes your boot partition once for that time you boot.

If you want to change it permanently, boot to OS X, open System Preferences → Startup Disk and select your OS X partition, or boot to Windows, open the Boot Camp Control Panel and select the OS X partition as the boot partition. Then, you will need to ⌥-boot to boot to Windows.