It's definitely a snapshot at a point in the past. There's no point in buying ETH now or moving off exchanges now... the snapshot is in the past:
"In OMG’s case, 5% has already been set aside from the total issuance (see sale doc), ready to be distributed by an automatic airdrop. That is, sometime quite soon (TBA soon), every address on the Ethereum blockchain that held a balance over a minimum threshold of 0.1 ETH at a recent past block height (TBA soon) will receive a share of this 5% that is proportionate to their share of ETH."
And it's not going to be a huge amount of OMG anyway, I work it out as $1 or so of OMG per 3 ETH (see my post earlier in this thread).
got to love free crypto. You tink OMG will crash a little after the drop?