The son of a Corsican woman who had married General Joseph Mordrelle (died in 1942), Olier Mordrel was born in Paris and spent most of his childhood there (paradoxically, the place where he also learned Breton).
Together with Roparz Hemon, he created the literary magazine Gwalarn (1925), and was included in the Breton delegation to the First Pan-Celtic Congress in Dublin (alongside François Jaffrennou, Morvan Marchal, and Yves Le Drézen).
The same year, he conceived the Strollad Ar Gelted Adsavet (SAGA, Party of Risen Celts) and its Nazi-like platform - which included Bretons in the Nordic "master race".
According to the architectural critic Daniel Le Couedic, the "gentle sweep" of the broad white bands around the corner is contrasted dramatically with the strong angular and stabilising vertical structures of the windows.
Several other Mordrel buildings are no longer extant, including his Garage d’Odet, a garage/factory designed to use modernist style to achieve space for maximum efficiency of manufacturing and repair.
A Lizer Brezel ("Letter of War") they wrote to PNB members in January 1940 stated that "a real Breton does not have the right to die for France" and "our enemies are first and foremost the French, it is they who have not ceased causing misfortune to Brittany".
After a self-designated Congress in Pontivy founded the Breton National Committee, Mordrel took charge of the PNB in late October, and subsequently led a campaign against Vichy France that was tacitly encouraged by the Germans.
In September Mordrel was allowed to return to Rennes, where, while both the PNB leadership and Vichy agents called on the Germans to expel him, he was kept as an alternative by the Nazi authorities.
He saved his life by telling everything he knew of his fellow Breton separatists, the members of the Irish Republican Army and relations with different German secret services.
Mordrel returned to France incognito in 1972 and continued writing for La Bretagne Réelle as Otto Mohr (a name he had used in 1940), as well as editing several books - including a history of the Waffen-SS (Waffen SS d'Occident).
In the 1980s, he was among the founders of Kelc'h Maksen Wledig ("Emperor Maxentius' Circle"), together with such figures of the far right as Yann Ber Tillenon and Georges Pinault; later, he was active in the GRECE, an organization associated with extremist politics, led by Alain de Benoist.
Mordrel's son Tristan Mordrelle (pen name André Chelain) is a far-right historical revisionist who runs the Association bretonne de recherche historique (ABRH) and edits its journal L'Autre Histoire.