[Galaxy S II General] Donaghy:
Almost. Official (stock) unbranded firmware is Android 4.0.3, the way it leaves the Samsung factory on the phones, before they reach the carrier. They are, however, not the same for all firmwares. There are different types of firmware, with different 'names'. Right now, the most recent one is the one we call LPG, which is different than the previous LPF, LPS, LP9, LP8, LP7 etc. The differences between these firmwares are updates: LPG is the newest, and as a result does not contain bugs that were present in prior firmware releases. They do look the same (more or less, minor changes notwithstanding), and are all part of the ICS Android 4.0.3 family

You can just forget about the CSC; it does nothing. CSC is your Country Sales Code, and has no influence on your device itself. If you flash a firmware with a different CSC, it should have no influence on your warranty, especially if you haven't rooted the phone. Like I said before: I handed my phone in with a Korean CSC code, and no one asked questions.

You are right about the reverting: if you flash back to the firmware you have now (the exact same one), no one will ever know something else was on your phone at some point.

It shouldn't. Warranty is a Samsung thing, not an O2 thing. Samsung don't give a damn if you have it unlocked or not.

It is probably just as safe, but the benefits of the Mobile Odin Pro app are:

1. You don't have to connect your phone to your pc (and because of that, you don't have to worry about anything happening to your cable, the right driver software, KIES being running in the background on your computer and fighting ODIN, etc.). You just put the downloaded firmware file on your SD-card of your phone, and flash the firmware ON YOUR PHONE. You can do it anywhere, at anytime. I've done it at bus stops, at the airport, at work. Takes all of 10 minutes. Just click 'Flash Firmware', leave your phone alone for 10 minutes and it's done.

2. You cannot make a mistake by opening the file in a wrong way. Even though if you follow the instructions for regular (computer) ODIN to a T, there are still people who manage to open the file in a wrong way and with that, screw up their phone. Mobile Odin gives you a one-click method, conveniently called 'Open File'. No need to worry about "do I open this in PDA? or in PHONE? or in .." well you catch my drift.

What I meant by 'it does everything' is: Mobile Odin Pro flashes the firmware for you, and if you check the box that says "Root phone using EverRoot", it will also automatically root the phone for you. No need to first flash the firmware and then finding and flashing the right unsecure Kernel.

There are more fun things you can do with Mobile Odin Pro (like Frankensteining your own firmware) but you don't need that right now.

Correct.

You need the one that says: Check Fus Downloader 2.1. If you click here, it should start downloading to your pc. Once you open CheckFus, follow the steps in the picture I attached (starting with 1, ending with 5). I've set it up with so that it displays the newest (LPG) version.

This downloads a file to your computer, which you than unpack with 7-zip (not with WinRar). It leaves you with a .tar.md5 file (long filename). This is the firmware file you will eventually use for your flashing. If you use Mobile Odin: put it on your SD card and open it within the app.

NOTE: you need a rooted device to use Mobile Odin!! If you are not rooted yet, flash the new firmware via your computer (see explanation in this thread), then flash the unsecure CF-Root kernel (found here). Explanation of how to do that here.

Be sure to follow all instructions in those threads and bookmark them. Good luck.

EDIT: Oh yeah, or what the guy ahead of me already said... Well, better twice than not at all.