I'm giving you guys automatic surprise. There are five archers (one is currently crazy) and two crossbowmen. Thurgan has a crossbow. I'm going to let Rhys turn and fire his bow, if desired, after the surprise round. I'm also going to say that you guys are going to open fire once the orcs are about 100 feet away. This will put all the missile weapons at medium range.
Okay, so here's what we're doing. There is a 1 in 6 chance the ogres will hear Severi before he has a chance to steal something from the ogres: 1d6=5
Further, I will make an intelligence check for the ogres. Success indicates they notice something is awry and stop before the oil trap. The rulebook gives their Intelligence as 6. They will succeed on 1-6 on 1d20: 1d20=3