How do I create oneworld account?
 

Is there something called oneworld account? If so, how do I create?

I'm not aware of any "alliance" (that's what oneworld is, after all) frequent flyer account in and of itself. Any status you achieve in your preferred carrier's frequent flyer account is recognized with the equivalent oneworld status.

May I ask... why do you ask? That is, what are you trying to accomplish?

What would you do with a oneworld account?
Mileage earning and awards are handled by individual airlines. There is no account with OW.

I once saw there is elite status with OW. Hence....

I recommend that you read the following pages from the oneworld site:
http://www.oneworld.com/ow/ffp/oneworld-status

(emphasis mine)
As a member of *a* oneworld® airline's frequent flyer programme...
(in other words, choose an airline, gain status on that member airline and you receive benefits throughout the alliance)

...

"Enjoy oneworld benefits with your own airline's frequent flyer programme"

AND
http://www.oneworld.com/ow/FFP-oneworld-benefits
"Although the oneworld alliance does not offer its own frequent flyer programme..."

Happy and safe travels

Just to jump on miamigrad's very complete answer: There are three OneWorld elite status levels: Ruby, Sapphire, and Emerald. OneWorld alliance members (e.g. AA) also have elite status levels. In the case of AA, they are Gold, Platinum, and Executive Platinum.

As per AA's agreement with OneWorld, AA's elite members also enjoy one of the three levels of status levels with OneWorld.

AA Gold is also OW Ruby.
AA Plat is also OW Sapphire.
AA EXP is also OW Emerald.

If you have OneWorld status (by earning status on one of the member airlines), you enjoy the privileges of OneWorld status on any member airline.

For eg. if I am travelling in Cathay, and I am a AA Plat. How do I enjoy the privileges? Should I give me oneworld account info to Cathay? How does it work?

There are no "oneworld" accounts, only individual airline accounts. The Cathay system may recognize your Plat/sapphire status when you make any reservation and input your AAdvantage account number; otherwise show them your AA Plat card, which has a little blue oval (sapphire) at the bottom which tells them your status. In my experience most Oneworld airlines' computer systems can tell your status from your account number.

The only "oneworld account" someone might have is if you make an online account for the RTW tool in order to save your planning online.

Everything else is done through the FF program of one member airline.

I have QF Silver FF membership. In theory, that means AA would recongise me as an AA Gold (both OW Ruby).

Simply put, just give your AAdvantage number to Cathay so that your flight on CX is creditted properly into your AAdvantage account. Then just show up at the airport and all your elite privileges that you're used as AA Plat will be reciprocally accepted by Cathay as their system automatically knows that AA Plat = OW Sapphire level; priority business class check-in (even when flying in economy), CX lounge access, priority boarding, etc.