You could make it take a really, really long time to find more obscure things. Everything that ever happened is a lot of data to sift through, so that should be reasonable enough. Like the internet, perhaps all of the major events could be well-indexed and easy to find. But the more specific and obscure things that an Oracle might want to know may be much harder to find. If the web page you're looking wasn't indexed by Google, it'd take a much longer time to find it, wouldn't you say? So as a limitation, just make it take an inconveniently long time to find things that are not the big moves and shakes of history. (This has also been brought up in this answer and this answer.)
Actually, this idea comes up in the manga ib: Instant Bullet by Akasaka Aka, and I think it presents the concept of the search duration limitation rather well (read right-to-left):