or does it mean that every 100 missed shots by his team while he is on the court, he will grab 20 offensive rebounds?
This is what the stat measures. Now, it doesn't exactly show up that way in the sim because there's not a nice, neat 100% between all of your offensive rebounders & all of the defensive rebounders. So it's really 20% of the cumulative between your 5 oreb% & defense's 5 dreb% plus whatever bonuses gained from having a "significant advantage" in 1v1 matchups & the penalty the shooter faces to their rebounding.
That's why more is better and you shouldn't aim for any specific number when it comes to rebounding.
Offensive Rebounding Percentage = 100 * (ORB * (Tm MP / 5)) / (MP * (Tm ORB + Opp DRB)). Offensive rebounding % is an estimate of the percentage of available offensive rebounds a player grabbed while he was on the floor.
Handy guide to figure out advanced stats