IMAC OSX install help?

[ISKOTB]

I got a couple of years old iMac that someone installed just windows on it and deleted the OSX partition!
I’m trying to make it boot from a Yosemite USB Ext drive install or even access CMD-R so I can format and restore it back but no luck. I thought the Apple BT keyboard and mouse isn’t connected so I tried a windows USB keyboard, still it just boots then after the grey screen it boots right to windows.

Any ideas please?

 

[Fishrrman]

Have your external drive connected and turned on.

Reboot and hold down the option key as soon as you hear the startup sound, and KEEP HOLDING IT DOWN.

The startup manager should appear, showing you all the volumes from which you can boot.

Do you see the external USB volume? If so, select it with the pointer and then hit return.

Any better?