As children, Ossifer and his brother, Maynard Plumm, visisted the Rock. Tyrius Lannister jested with Maynard and told him that the ancestral blade of House Lannister, Brightroar, was kept at the highest point of the Rock, atop one of the balconies of the ringfort that could only be reached by climbing. Maynard believed him and crippled himself attempting to climb to the balcony.
Ossifer was present at the marriage of Lord Eon Crakehall at the Rock. While there, his wife, Lady Cyrenna Plumm, convinced King Damon Lannister to grant their daughter, Joanna Plumm, the position of handmaiden to Queen Danae Targaryen.
"The man can’t tell his pen from his prick, though I imagine the quill would be longer. He can hold a grudge longer than any woman I know, and I know very grudgeful women, myself included." - Lady Jeyne Lannister speaking to Queen Danae Targaryen
"Ossifer was a jealous little weasel, always envious of my brother especially, and when Tyrius called his banners to Pyke the Lord Plumm left him to die there.” - Lady Jeyne Lannister speaking to Queen Danae Targaryen
Maynard Plumm, brother
Cyrenna Plumm, wife
Philip Plumm, firstborn son
Joanna Plumm, daughter
Edmyn Plumm, second son