How do i reinstall mac osx on a mac that is currently running windows?

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There is a problem, somehow when I was installing Windows 7 on my Mac I lost the Mac OS. When i boot up and hold the alt/option key it only shows the option for Windows. How do i get OS X back?
 
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Your Mac's Specs
Silver M1 iMac 512/16/8/8 macOS 11.6
G'day and welcome to the forums.

Sure you installed Windows 7 on a partition and what does Disk utility say about the hard drive? Wipe the hard drive and install OS X and then Windows again via Bootcamp if it is not there.
 

chscag

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2017 27" iMac, 10.5" iPad Pro, iPhone 8, iPhone 11, iPhone 12 Mini, Numerous iPods, Monterey
First, make absolutely sure that you accidentally overwrote OS X before proceeding. If you did overwrite OS X, you will have to reinstall from your original install DVD set.

Follow along: Step one: Reset the PRAM|NVRAM. See this LINK.

Step two: Boot the machine with your original install DVD. Remove the Boot Camp partition and then drag the partition line until it fills the entire hard drive and you have one partition.

Step Three: Erase the hard drive and format it as HFS Extended Journaled using the GUID scheme. Name the hard drive "MacIntosh HD". (no quotes)

Step Four: Reinstall OS X and all your applications.

If you have a Time Machine backup you can use it to restore everything during the reinstall process.

As I said above and reiterate: Make sure that you overwrote your OS X installation before proceeding.
 

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