I've got my Macbook Pro (early 2015) set up with Boot Camp. Normally, when I switch it on, OS X starts. But I can press Alt/Option to get the boot menu, and select Boot Camp Windows from there.
Once I've booted into Windows, though, Windows becomes the default OS! So, after booting Windows, then shutting down, when I next switch it on, Windows starts. If I want OS X back, I have to select OS X from the boot menu, or use the Boot Camp section of the Control Panel to change the boot option.
Is there anything I can do about this? I want my computer to always boot into Mac OS X, even if I last opted to boot Windows, unless I go via the boot menu. My old Macbook Pro (mid 2009) worked that way, and I've come to prefer it.