IMac – Can’t get iMac to stop booting Ubuntu

I've just inherited an iMac from a departed coworker. It is a 27" unibody model with sandy bridge i5 (not slim, so, from mid 2011).

It came automatically booting Ubuntu 18.04.1 LTS as its default OS when it starts up. I've never used Ubuntu before, but I was able to find my way around enough in the file explorer to find that there appear to be two hard disk partitions: one for Ubuntu, and one for OSX.

I would like to boot into OSX, but none of the methods I've found from googling seem to work.

Most notably, I can't get seem to get into the startup manager. When I restart the computer, there is no chime that I can time my pressing and holding of the "option" key to, but I've tried it many times waiting various amounts of time after the grey screen comes up, and no matter what, it always just boots Ubuntu. When it restarts, it goes to a grey screen for a few seconds, then do a dark burgundy screen for a few seconds, then back to a grey screen for a few seconds, before the Ubuntu startup screen appearing.

I don't know any details of how Ubuntu was installed, if it's an EFI setup, etc. What's the best way for me to figure out what's going on, and how to get it to boot into the OSX partition given that Ubuntu is currently the default, and I can't get the startup manager to appear?

I'm using a bluetooth keyboard, I've read about people having issues getting startup manager to appear when using a BT keyboard and that they have to wait until after the startup chime ends to press and hold the option key. But, again, I have no chime on startup.

Thanks for any help that anyone can give to see what's going on!