Book Ned was implied to be an "okay" swordsmen he wasn't mook status but he wasn't great. He's known more for his war tactics, pretty much the brain behind the rebellion along with Jon Arryn. The only notable fight Ned has had was against three of the gawdbody KG where Ned was rolling seven deep anyway and he still would have died if it wasn't the Howland Reed assist.
Jaime was co-signed by two of the best KGs. Even crossed swords with a Mountain type dude when he was a teen. If I remember correctly, Jaime's thoughts shows that he was confident enough to take out The Hound and The Mountain if he had to fight them one on one because they didn't really have technique just crazy brute force that he could get around with careful speed and his superior sword skills. This is arrogant Jaime thoughts though.
As for Oberyn he mains a spear (which is an easy to use noob weapon) and depending on the combat setting a spear >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> sword. He could very well put a two handed Jaime in a body bag. Helps that he uses poison too.
Yeah, Mountain's head was sent to Dorne. Qyburn's experiments doesn't need the head
by the time the book starts.
