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.

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).

The creature gets the counter if its copiable characteristics as it would exist on the battlefield include the specified creature type. For example, say you control Conspiracy, and the chosen creature type is Rogue. If you put a creature onto the battlefield that isn't normally a Rogue, it won't get a counter from Oona's Blackguard.

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.

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.