I'm A Banana
Potassium-Rich
Three tactics used were 1st... to blunder into its lair and confront it in a water mineral laden cave fulll of 'mites that inhibited any large weapon used (the Orges club, the Rangers Greatsword) and all missile weapons. This resulted in specacular modern art on the already wet walls using the PC's innards as media.
So...they were forced into melee with sub-optimal weapons and little room to maneuver, against a giant melee-damage monster? The ogre seems to have things stacked in his favor, here...
Second they hid the rouge in some roots near the entrance and called it out, cast entagle its save was an 18 (all rolls were open as this was a test of the system) and fortunatly the rouge saved as well. It them charged out and dined on the PC's buffet style.
There's room for the 20' run to charge, but no room for ranged weapons? Ehhh?
Third tactic, after the group had been remade into 1st level killing machines was to again cast entangle on it (again it saved with an 18) and then to missile it with bows why a spell buffed ranger (prot from evil and sheild) fought it up front. They had a hard time with the 16 AC (the rolls were unimpressive to say the least) and after three rounds the Ogre managed to hit the Ranger. As observed by the previous posts, the ogre one hit killed any PC in the party.
....and then they had some bad luck?
Seriously, CR is not an exact science. If the ogre was rolling in the 10-20 range but no PC beat a 12, I think blaming the system used to approximate challenge is off base to say the least.