For any dealer in OTC options, a short option position represents a liability, since, after receiving the initial payment, that contract offers only potential cash outflows.
To use an IFRS definition, a liability is:
I'd expect these liabilities to be recognised at "fair value through profit and loss", i.e. variations in the option liability would generate profit or loss.
To emphasise the point: IAS32 defines a "financial instrument" as "a contract that gives rise to a financial asset of one entity and a financial liability or equity instrument of another entity".
The hope of the dealer is that the option premium received is sufficient to cover the costs of hedging that position plus a profit margin.
 Ignoring any margining / collateral-type cash flows.
Edit: have a look at Deutsche Bank's 2015 balance sheet at https://annualreport.deutsche-bank.com/2015/ar/financial-statements/consolidated-balance-sheet.html
If DB had sold you an OTC option, I'd expect the option position to be reflected in the line "Financial liabilities at fair value through profit or loss", in the sub-item "Negative market values from derivative financial instruments".