The note you found implied that a traitor exists among Ordlac's retainers.
'If indeed one of my retainers is a traitor, this could bode ill for any effort we make to bring aid to Stoke. I will continue my muster, but I must ask you to investigate this villainy.
'If you would, speak with Cenulf, one of my most trusted lieutenants. I have need of Dudsig's assistance, but Cenulf can guide you during your investigation.
'You may find him near the mill here in Oserley.'
Cenulf may be found near the mill in Oserley.
Thane Ordlac asked you to speak with Cenulf and obtain his guidance during your investigation of the brigands' note.
Mounted half-orc archers can be found throughout the fields west of Oserley.
Cenulf asked you to defeat mounted half-orc archers to discover more evidence of the traitor in Oserley.
Cenulf is near the mill in Oserley.
You should return to Cenulf with news of your victories against the half-orcs.