He is married to Ida d'Avesnes.
They got married
Child(ren):
FASTRE [I] d'Oisy, son of --- (-before 1092). Avoué de Tournai. The Liber de Restauratione Sancti Martini Tornacensis names "Fastredum eiusdem urbis advocatus" when recording his marriage[750].
m IDA d'Avesnes, daughter of WEDRICUS d'Avesnes & his wife ---. The Liber de Restauratione Sancti Martini Tornacensis names "Idam, germanam Theoderic de Asvensis" as wife of "Fastredus"[751].
Fastre [I] & his wife had [four] children:
GOSSUIN d'Oisy (-1127).
FASTRE [II] d'Oisy (-after 1111).
ISEMBARD (-after 1117).
Daughter Nn
https://fmg.ac/Projects/MedLands/HAINAUT.htm#AdelaideAvesnesMEngelbertEnghien