When Catelyn negotiates with Lord Frey to gain permission for Robb and his army to pass the Twins, one of Walder's demands is for his son Olyvar be made Robb's squire, and eventually be knighted. Robb accepts these terms.
In the books
In the A Song of Ice and Fire novels, Olyvar is the eighteenth son of Lord Walder overall and the fourth born to Lord Walder's sixth wife, Bethany Rosby. His three older brothers are Perwyn, Benfrey, Willamen, and his younger sister is Roslin. He is two years older than Robb, although he is very immature compared to Robb. He takes part in the Battle of the Whispering Wood.
Olyvar, alongside his brother Perwyn and nephew Alesander, are kept in the dark about Lord Frey's plan to betray and kill Robb at the Red Wedding. Apparently it was suspected that the three of them were too loyal to Robb, and would try to warn him or even to defend him when the ambush commenced, so they are sent away from the Twins "on duty". Catelyn and Robb ask about the absence of those three Freys, but receive only evasive answers. Too late, as the massacre begins, Catelyn realizes the significance of their absence.
|Seat:||The Twins · Riverrun||Lands:||The Riverlands|
|Titles:||Lord of the Crossing · Lord of Riverrun · Lord Paramount of the Trident|
|Current members:||Arwaya Frey · Derwa Frey · Freya Frey · Janeya Frey · Marianne Frey · Merry Frey · Neyela Frey · Olyvar Frey · Roslin Tully · Sarra Frey · Serra Frey · Shirei Frey · Walda Frey · Waldra Frey · Waldron Frey · Ryger Rivers|
|Deceased members:||Joyeuse Frey · Lothar Frey · Walda Frey · Walder Frey · Walder Rivers|
|Household:||Malcolm · Talbot|