2008-04-01 : If Oona’s Blackguard enters the battlefield at the same time as another Rogue (due to Living End, for example), that creature doesn’t get a +1/+1 counter.

2008-04-01 : If a Rogue would normally enter the battlefield with a certain number of +1/+1 counters on it, it enters the battlefield with that many +1/+1 counters plus one on it instead. If a Rogue would normally enter the battlefield with no +1/+1 counters on it, it enters the battlefield with one +1/+1 counter on it instead.

2008-04-01 : The creature gets the counter if it would enter the battlefield under your control. It doesn’t matter who owns the creature or what zone it enters the battlefield from (such as your opponent’s graveyard, for example).

2008-04-01 : The effects from more than one of these cards are cumulative. That is, if you have two Oona’s Blackguard on the battlefield, Rogue creatures you control enter the battlefield with two additional +1/+1 counters on them.