Teplin Macagan is a dwarf located in Oo'glog, near the bank. He can decant 1 to 6 dose potion flasks into either 6-dose potion flasks, or 4-dose potion vials; or he can decant 1 to 4 dose potion vials into 6-dose potion flasks.
You must bring extra flasks as Teplin will not provide them, though he does provide vials. The tasks can be performed on noted or unnoted flasks and vials.
You must have completed the quest As a First Resort in order to bring him to Oo'glog.