You can wait until the OS commences. Important thing is that boost is active at the moment of reaching the enemy.
On a side note: this also goes for for example yieldig boost. If boost runs out *after* your troops are sent and *before* they reach layer, the boost is not active on that layer.
* Attack, defend, yield: before reaching target.
* March, build, learn: before starting process.
* Will to Live: before troops are killed.
In an OS one typically make sure everyone has put boosts on in chat – then initiator execute the onslaught and boost the march.
How ever, even then it is not certain you won't waste boosts:
[borkof]: Anyone else want to participate?
[simla]: No, let's go
[gandalf]: I want, wait for me to change shaman
[simla]: hurry up, my boosts are running
[gandalf]: I'm in
[borkof]: Everyone boosted and in attack gear?
[yodol]: Oh, crap, I'm in building gear
[simla]: f***! Only have 3m left on boosts.
[yodo]: OK. I'm ready
[simla]: I need to put on new boosts
[simla]: OK, ready
[miimi]: Can I join?
[simla]: …
- 3 minutes later -
[borkof]: OK. Everyone boosts on, Here we go!
[simla]: hahah he shielded