One hit from Oghren and Varric is down. Varric has Bianca and is an experienced rogue but he actually has to stay away from Oghren in order to win that fight, while Oghren just charges him down.
We all tend to forget the fact that Varric is very squishy and has pretty bad dexterity for a rogue, and is a surface dwarf on top of that. Oghren is from the warrior caste of Orzammar, he's grown up his entire life being taught how to fight and kill and has spent much of his life fighting darkspawn in the Deep Roads like all dwarven warriors do. Not to mention other dwarves in Provings. Oghren is no pushover and even other dwarves in Orzammar acknowledge that he's a great warrior who simply went down the drain because of drinking and Branka.
Oghren is strong, there's no denial in that. But he also isn't much of a big deal, considering out of the characters he's one of the least tatical reasoning.
Varric, on the other hand, although not as physically strong as Oghren, counts with a unique weapon (Bianca) and a lot of skills that are pretty much tatical reasoning - like the traps, for example.
If Oghren got to catch up with Varric, he would deal some devasting blows, but I do think Varric would notice that if he tried to keep to close to Oghren he would perish; using all the abilities he has, Varric would keep his distance + using the traps, poisons and other methods to keep Oghren at bay while making it rain arrows from Bianca.