Anything but North ;-)
I love the spunky Oliver. I think these names from your list would be fine for his siblings.
Other Suggestions: Sebastian, Felix, Hugo, Julian, Theodore, Lucian, Simon, Laurence, Everett, Thomas, Leo, Benedict/Bennett, Vincent, Maxwell, Gideon, Tobias, Miles/Milo, Gabriel, Emmett, Barnaby, August, Tristan, Edmund
Other Suggestions: Sophie, Alice, Eva, Charlotte, Georgina/Georgiana, Daphne, Clara, Cora, Cecily, Grace, Isabel, Eliza, Josephine, Louisa, Helena, Violet, Lydia, Phoebe, Delilah, Bridget, Fiona, Felicity