Is there any Mac Operating System available to download ?
The short answer is, as others already said, there is no free OS X.
The long answer is this: An operating system consists of many parts and large parts of OS X are indeed free. If you are interested in this have a look at the Darwin project.
What is missing from Darwin is mainly the graphical user interface of OS X (Aqua). You can run GNOME or KDE instead if you like. One of the consequences is that you can't run applications that depend on Aqua, like Safari or iTunes.
And just not to give false hopes: Darwin is very cool if you are interested in operating systems but in my opinion it's not very useful for most practical purposes.
OS X updates tend to be free - but IIRC its between consecutive versions, and you need to have apple hardware, or a running system. There's no way to 'simply' download an ISO and install it on an arbitrary system. Annoyingly for a reinstall there's no 'iso' download - you could in theory build an installer off of an ESD if you did have a mac and internet recovery is your best bet/supported bet for a bare metal install.
I'm assuming that's not what this is about. You'd want to run this on arbitrary hardware.
As of 2016. Nope. There's been a few attempts - you could get the kernels free for a while (there seem to be versions here), and attempts to throw together a functional userland around that - puredarwin at least has a webpage, but is not really there.
Kernels are lovely. Also entirely useless on their own. Get a usable userland and you can run the otherguy's stuff. For that matter, apple keeps its userland close to its chest, and that's the big pain point. Darling might help with that, you'd essentially need to port that back to darwin, which would be wierd. You can't simply download the GUI elements of OS X in any way.
In short, the fragments of a usable 'free' OS X 'compatible' system exists, but is in no way usable, and there's no official legal gratis release of OS X. There's pirated/hacked ones, but those are out of the scope of this site.
Nope, sorry. However, you can (questionably legally) buy a copy of Mac OS X and install it on a PC ("hackintosh"). Also, keep in mind that new Macs come with Mac OS X.
Based on what I read on the Apple forums, if your Mac is really old (mid 2009 or older) you can use the supplied installation discs to reinstall the OS.
If you don't have the install discs, you can contact Apple and they'll ship it to you for a small fee. After installing an older version of OS X, you can then upgrade to El Capitan if your Mac meets the hardware requirements.
Alternatively, if you have access to another Mac or can borrow one from a friend, you can create a bootable USB recovery drive using OS X Recovery Disk Assistant and use it to reinstall Mac on your machine.
No. Mac OS X costs $129 USD and is available through Apple. While you may be able to find some locations that sell it for slightly less, you will not be able to obtain it for free unless, say, a friend gives it to you.
Also, please keep in mind that according to the License Agreement (or whatever), you can only install Mac OS X on Apple computers. (But it's possible to do so on other computers, just not legal).
Get up to 7.5.5 from here.
(I am not a Mac user, but I believe I can point you in the right direction.)
I understand you are looking to install OS X on a bare-metal MacBook that you brought back to life. I assume here that you are having this difficulty because you are using Windows.
El Capitan is the latest OS X version. You may download a DMG file of it from Getintopc. I have verified that the download starts, but as its size is 5.7 GB, I did not wait for it to finish. The same website also has other downloads for previous versions of OS X.
To install it, the next step is to convert the DMG to a bootable format. See this post for doing that: Create a bootable USB drive from a DMG file on Windows.
In case of a problem with the above, the DMG file can also be converted to ISO and then to a bootable CD/USB. See this article: How to Convert DMG Files to ISO Files on Windows.
Once you have the bootable CD/USB, the next step is to boot and install. However, not posseding a Mac, I cannot help you with this.
You could also try first to install the bootable media in a virtual machine by using VirtualBox, VMware or Hyper-V (if you have Windows 10). This exercise may help you to prepare for the real thing.
The original question does not give any details as to the purpose or target hardware of the OS.
Apple stopped charging for OS X as of 10.7 (Lion). They also stopped pressing physical media at that time. The ONLY authorized distribution channel for OS X 10.7 and higher is the Mac App store.
For computers that shipped with 10.6 (Snow Leopard), the App Store must be downloaded and installed as a separate application to gain access to Lion and higher (which is free).
For computers that shipped with 10.5 (Leopard) and lower, they must upgrade to Snow Leopard first (which is NOT free), so that they can download the App Store app and then upgrade to Lion or higher.
So the answer to the question is yes, there is a free version of OS X available from Apple IF your Mac originally shipped with Snow Leopard or higher. Otherwise, you must pay for Snow Leopard (which Apple no longer sells) to do the upgrade.
And since the EULA prohibits OS X from running on non-Apple hardware, the answer is always no if you don't actually own a Mac.
Mac OS X is free, in the sense that it's bundled with every new Apple Mac computer. But it's not freely available.
Search for "hackintosh". There are people installing unsupported versions of the operating system.
Depending on your hardware age and how updated it was when it last had a drive, yes. Boot holding Command+Option+R.
This boots to internet recovery, which downloads an image that allows you to reinstall OS X.
Interesting that nobody has mentioned, that GNUStep is largely compatible with Cocoa, i.e. the API for MacOS. But it is more of interest to programmers, I guess.
You can always click on the headers to go to homepages for updates. Download links in this answer (no-hassle mode :P).
No, Hackintosh won't work. It needs Apple hardware, and is illegal.
There is no legally free version of OS X, but you can take a look at Darwin. (Didn't link to Hackintosh on purpose, SE and SU are not intended to spread piracy. I don't know if PS (Pirated Software) StackExchange is ever going to exist.)
PureDarwin is a project meant to continue where OpenDarwin left. It's latest version, PureDarwin Xmas (booting process fixed), still in development (?), is available as a VMWare virtual machine disk (PureDarwin 9). Their website also offers a "nano" version (PureDarwin Nano) as both an ISO file and a VMWare virtual machine. They also link to an old Darwin 8 ISO file stored on apple's Darwin website (actually, they link to the source browser).
Apple used to store compressed ISO files of Darwin till version 8.01 (linked from PureDarwin's site).