The O/U came about in the early twentieth century. Since then it has become the weapon of choice in almost all competitive shooting disciplines and is a favorite for many an upland pursuer. The appeal of these guns is largely due to reliability — pull the trigger twice and the guns goes bang twice (we won’t even begin to wade into the single vs. dual trigger arena, head to the Lodge to discuss that).
The stacked barrels create a a more narrow sighting plane when compared to the side-by-side, which nets more precise long shots.
One of the largest hunting advantages of the over-under is the ability to have both a ‘near and far’ choke — two separate chokes allow for different shot spread and effective distance — first shot for up close and second shot for longer yardage.
As with other break action weapons, safety is always touted as one of the biggest benefits. An opened action is a visible sign to guides and other hunters alike that the weapon is secured. And the ability to easily look through both barrels when open allow hunters to recognize any barrel obstructions or fouling. This also makes the barrels super easy to clean, both in the field or on the workbench.
Besides the need for a PHD to fix the O/U in the event of a rare malfunction, the only other negative often espoused is sticker price.