Does anyone have any ideas on how an oblitiator (sorry if spelt worng) could be used as a player class.

Well Black Crusade not really has any sort of class system just a starting archetype set. So after you start the game for example as nurgle marine you do your job, meet new interesting people you got introduced to an Obliterator cult. It is love for the first time true and wonderful so the elevate your rotten meat-shell to a glorious new form. The end.

Yep. I know. You become infected with a virus.
An example.
http://darkheresy.wikia.com/wiki/Daemonic_pacts

My acolytes got into both of them Bride of malal and obliterator virus.

Obliterators, IIRC are introduced within the Tome of Decay as an NPC Elite (Tome of Decay P93). General fluff though is that it is tech marines which have been exposed to the Obliterator virus and have become one with their weapons and tech. There are various stages to how this infection takes hold which could give you an IC way of handling this. Reference: Lexicanum which starts off with minor improvements to their skill with firearms and weapons. Before long they start to find themselves merging with their guns, generating the ammo for it, their skin melting into their armour and muscular mass bulging out to support the plethora of weapons available.

As a player class, that might be a tough one to go with though. Generally they are bound into their armour and have almost any weapon that can be envisioned within the Imperial (and Chaos) arsenal projected from their arms. So a figure starting in terminator armour, high toughness with access to things like Las-cannons, multi-meltas, power fists, plasma weapons etc is a very powerful character class. This would need to be something which is an elite advance or something which someone develops into or starts at high level.

As a rough idea though without giving too much away from the book, Obliterators are pretty slow and ponderous though so you'd need to nuke their agility to the ground and give their Strength as well as Toughness a boost to be able to support the weaponry. What normally compensates the obliterators terrible movement speed though is that many are fused in their terminator armour which of course means teleportation methods to get closer to the fight. In terms of extra equipment, they never need a weapon as they can generate anything that they ever need and never need to reload. This will become particularly potent in a prolonged firefight and can neither equip nor unequip any armour or weapon and likely can't use a whole lot of other gear like tools either so they might be useless.

Hopefully that page reference and the above idea will give a good starting point but I would highly recommend caution. This will prove to be a massively potent weapons platform and that's it. They will find it nigh on impossible to do anything intellect based, fellowship based, mobility based...so if your campaign will involve these, decide carefully before you say yes to the player on this or you may be condemning them to a death which they cannot dodge out of the way of (literally, since their Ag is crap).

I also would not recommend creating full blown obliterator.

When we made an Obliterator - mind you, it's only been a handful of sessions, so it's not fully tested - we gave him special Talent, which allowed him to "store" weapons inside of him, creating ammo and such. It looked like this:

- Obliterator can switch the weapon he's using as Free Action.

- When he runs out of ammo in a magazine, he needs to replenish it by switching for different weapon for at least 1 Turn.
- Weapons assimilated by Obliterator cannot be retrieved - they are now part of him forever.

- Assimilating every Weapon costs 200 xp and it gets more expensive each time, so first weapon is for free, second for 200, third for 400, fourth for 600...

- For balance reason (and because he instinctively tries to assimilate every weapon he comes to contact with), Obliterator can only use his built-in weapons. If he tries using other weapons, they are either assimilated, or break as a result of failed assimilation. This way, he will not stick to normal weapons and assimilate only plasma/melta.
- Daemonic Weapons cannot be assimilated. Because of that, they can be used safely. By "safely", I mean they do not break, but they still hate you and want your souls.