Apple - How do I set OS X to boot by default?

You don't need to boot into OSX to change the default boot system ...

1. Hold down the Option key when booting, and when you see the system choices
2. Select your desired default boot device, then hover mouse over the up-arrow ↑
3. Hold down the Ctrl key and you should see the ↑ icon change to a "power on" icon
4. Left click on that "power icon" and that system will boot and that system will also be your default boot system.

Older versions of Boot Camp are similar, but I don't remember if the icon changes.

Official documentation here:

If you press and hold the Control key during this step, your selection is saved in Startup Disk preferences, so it persists until you change it.

You can set the default boot disk in System Preferences>Startup Disk.