reinstalling mac os on late 2008 imac with ubuntu already installed on it on it

You can use an OS X 10.6 Snow Leopard upgrade DVD to get a copy of macOS back onto this iMac. After installing OS X 10.6....make sure to fully update it with all patches. Then you can use the instructions in this Apple article to download a later version of macOS (you may need to first upgrade to 10.11, before going to 10.15):

How to download and install macOS - Apple Support

If for some reason you cannot download the macOS 10.11 installer when running OS X 10.6, then you can download the macOS 10.11 installer using any other computer since that installer is available outside of the App Store. Then transfer the file to this iMac so you can follow the instructions in the above article I linked so you can extract the real installer to the Applications folder where you will then run the "Install OS X El Capitan" app.

This method will WIPE ALL Data and there is No Recovery - Period.

>> Only works on Intel Based Apple Computer <<

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

Hi Teky_T,

In that case, you could try going to an Apple Authorized Service Provider to see if they'd be able to assist you in creating one. You can locate your nearest service provider and set up service by using the site: Find Locations

Cheers.

I must have mixed up my replies since I made a slight mistake in my previous post here. A 2008 iMac is only able to install up to macOS 10.11 El Capitan. None of the current versions of the popular browsers support macOS 10.11 anymore although you may be able to find a variation of these browsers which have been backported to support older versions of macOS (perhaps with more limited ability to render some websites).

Also, if you reinstall OS X 10.6, then you will need to properly partition & format the drive by using the instructions in this article (later versions of macOS 10.11+ changed how this is done):

https://eshop.macsales.com/tech_center/formatting/Mac_Formatting_6-10_R3.pdf

Also, you mentioned corruption with the previous macOS...personally I would first make sure the hard drive is still healthy by using Ubuntu to check the health of the drive by using GSmartControl app....this app may already be installed by default, but if not it is easily installed from the Ubuntu repository. Feel free to post the complete text health report here for examination.

Hello Teky_T,

If you can create a bootable installer, you may be able to install macOS. These steps contained at, Create a bootable installer for macOS - Apple Support will walk you through the process.

Be sure to follow the steps exactly as outlined.

Have a good day.