This just further showcases the fact that Laurel matters very little to Oliver in his life. Also, Laurel has been such a big part of Thea's life, that she literally only lists one thing, when she tries to talk Oliver out of dropping her as a friend. And conveniently ignoring that at one point she tried to give her mother the death sentence. Oh yeah, she was a great big sister. And it's a mistake he's owning up to. Since Season 4 started, he's actively trying not to keep secrets from the people he cares about anymore. Maybe. But Oliver had a very good reason not to tell her. Because he knew exactly what Laurel would do with that knowledge.