The Mac hardware stores the boot volume in NVRAM so you could power off the computer and then reset things by holding the Command Option P R keys down.
Once you've heard two boot chimes you can release things and if the Mac OS X volume is viable it will boot first. If that doesn't work then it's likely that the OS X volume has issues and the system is failing to boot there and falling back to the second viable OS - Windows.
You might be able to see that in the console if you boot verbose or single user mode.
The official fix at that point would be to boot into Recovery and then see about repairing or reinstalling OS X.
I was not able to get my Mac (2018 Mac Mini) to boot to anything other than Windows using any of the keyboard options listed here. In my case it's extremely likely because I'm using a third party keyboard that isn't activated until past when the boot override input is needed. I also didn't have a Mac keyboard or really any other keyboard immediately handy. However, what did work for me was the following:
If you have BootCamp drivers successfully installed you can access through the system tray in Windows a boot menu which allows you to force restart into MacOS. In my case the drivers hadn't installed correctly such that the boot camp menu did not appear in the system tray, but instead I was able to extract the contents of the Bootcamp.msi download into C:/Program Files/Bootcamp and then run the boot manager exe directly from Windows Explorer. After running the boot manager I was able to select MacOS as the startup disk from Windows and subsequently successfully booted into MacOS.