NOS tokens are airdropped to all Nano holders at a rate of 10:1. The snapshot was taken on November 13.

NOS is a utility token of a new DAG-based stablecoin projekt. It is forked from Nano similar as Banano.

From the NOS announcement:

Am I eligible for the airdrop? You are eligible for the airdrop if you held Nano during the snapshot taken on November 13. To make sure that only rightful account owners receive their NOS tokens, only users who are in control of their private keys are eligible for the airdrop. This means that exchange accounts like Binance or OKEx are excluded from the airdrop. We reward all accounts that have broadcasted at least one block to the blockchain and contained at least 1 NANO of funds in total (regardless if pending or pocketed).

Claiming the airdrop is quite easy, do it here.

If you need help, find more details here in a medium article.