Edit 2016: This is true for any OS except El Capitan - Apple are leaving it available so devices ineligible for Sierra can still obtain it. You need to already have it in your Purchase History, otherwise once it's gone from the App Store, that's it, it's gone. Sierra is still available from here, How to download macOS Sierra - Apple Support. If you know of a mac that can only upgrade as far as El Capitan download it on that mac, before it goes to the installation process make a copy of Install Mac OS X El Capitan.app which will appear in the Applications folder. And coy it over to your mac.
Apple Support El Capitan Download Free
January 27, 2016 fixes two more printer driver links on Apple's download page to support El Capitan. I've indicated the latest release date of drivers that I could find. September 30, 2015, 10.11 was released, and older drivers may not work with it.
In addition to offering full iTunes support for iOS 9 and amending parts of the app to fit in with OS X El Capitan, iTunes 12.3 also brings support for Apple’s new two-factor authentication process that has been designed to further secure a user’s Apple account and associated Apple ID.
To get the latest features and maintain the security, stability, compatibility and performance of your Mac, it's important to keep your software up to date. Apple recommends that you always use the latest macOS that is compatible with your Mac.
Find out how to upgrade to macOS Big Sur, the latest version of macOS.
Check compatibility
Does Apple Support El Capitan
The version of macOS that came with your Mac is the earliest version it can use. For example, if your Mac came with macOS Big Sur, it will not accept installation of macOS Catalina or earlier.
If a macOS can't be used on your Mac, the App Store or installer will let you know. For example, it may say that it's not compatible with this device or it's too old to be opened on this version of macOS. To confirm compatibility before downloading, check the minimum hardware requirements:
macOS Catalina 10.15 hardware requirements
macOS Mojave 10.14 hardware requirements
macOS High Sierra 10.13 hardware requirements
macOS Sierra 10.12 hardware requirements
OS X El Capitan 10.11 hardware requirements
OS X Yosemite 10.10 hardware requirements
Download macOS using Safari on your Mac
Safari uses these links to find the old installers in the App Store. After being downloaded from the App Store, the installer will open automatically.
macOS Catalina 10.15 can upgrade Mojave, High Sierra, Sierra, El Capitan, Yosemite or Mavericks
macOS Mojave 10.14 can upgrade High Sierra, Sierra, El Capitan, Yosemite, Mavericks or Mountain Lion
macOS High Sierra 10.13 can upgrade Sierra, El Capitan, Yosemite, Mavericks or Mountain Lion
Safari will download the following older installers as a disk image named InstallOS.dmg or InstallMacOSX.dmg. Open the disk image, then open the .pkg installer inside the disk image. It will install an app named Install [Version Name]. Open that app from your Applications folder to start installing the operating system.
El Capitan Download For Mac
macOS Sierra 10.12 can upgrade El Capitan, Yosemite, Mavericks, Mountain Lion or Lion
OS X El Capitan 10.11 can upgrade Yosemite, Mavericks, Mountain Lion, Lion or Snow Leopard
OS X Yosemite 10.10can upgrade Mavericks, Mountain Lion, Lion or Snow Leopard