Can't boot to Mac OS after inserting Win10 HDD

[arkviz]

Recently I have to inserted functional Windows 10 HD from another Mac Pro 5,1 into one of empty HD bays inside Mac Pro 2,1 2007. I didn't touched Mac OS Drive. Selected Windows volume as startup disk, restarted and Windows successfully worked. But now i want to start Mac OS on another drive and I can't figure hot to do it. After start i hear chime. Holding option key, cmd+alt+p+r, cmd+v, shift... doesn't do anything. It always boots in windows. So i have physically removed Windows HD from computer. What I got was black screen with message "insert disc and pres any key". Removing all drives was showing more Mac-like icon trying to tell me, boot disk is missing. I tried Mac OS USB installer, not recognized. Reset SMC, nothing helped. Still only Windows is bootable. It looks, Win has reformated something at startup.
Can someone help me? I don't want to stay on Windows...

 

[CoastalOR]

Try booting to Mac Recovery (Command + r). Then try setting the Startup disk (upper left drop down once you're see the Mac Utilities) to the Mac HD.

 

[arkviz]

Thank You. This didn't hepled. But... Finally I've found a problem. Computer is OK, however, original 13 years old Apple wired keyboard was problem. It simply didn't woke up at boot chime, but slightly later. Replacing keyboard for another, yet older one, solved this problem. I was afraid of that Windows broke EFI boot or firmware or something like that.