The gendarmerie commander is the only officer who held the Obo brigade, in addition to a detachment of the Central African Armed Forces that is there.
The gendarme, who has been posted in the city for nearly three years, was killed at his home by armed men who attacked the city on Tuesday evening.
The violence that reached the city centre began hours earlier in nearby villages.
Villagers, hunters and traders have been murdered by armed groups likened to Fulani herdsmen present in the area.
Faced with an increase in acts of violence, the population of Obo last week held a march demanding that the government and UNMISCA take action to prevent them from reaching the city centre.
The town of Obo, capital of the Haut Mboumou prefecture, is near an area where Ugandan LRA rebels are losing ground inside their country.