According to the Whitepaper:
"Once per quarter starting from 01.01.2018 and continuing for 24 months, 2,000,000 tokens will be distributed among all holders of OTN tokens, in proportion to their stake relative to all emitted OTN tokens on the day of distribution."

An OTN token holder is a person who has either 'mined' their OTNs on the IQ Option platform or purchased OTN tokens on one of the exchanges.

You do not have to necessarily keep your OTNs on the IQ Option platform to benefit from the airdrop.

IQ Option will not be issuing the OTNs for the airdrop.

IMPORTANT NOTE:

All tokens belonging to the Open Trading Network Foundation will be EXCLUDED from the airdrop. Those are:

• The Open Trading Network Foundation's own OTN tokens
• The tokens reserved for OTN.Wallet motivation rewards
• The 16mln OTN tokens reserved for the airdrop itself.

UPDATE:
It's important to note, that those keeping their OTN tokens on the crypto exchanges will not be able to benefit from the airdrop. Please, withdraw your OTN tokens to an erc20 wallet by December 31st, 2017.