There isn't a very good search function here, so it takes a lot of patience to wade through the available materials. But like the 49'ers themselves, your tedium may be paid off with some valuable nuggets.
Search on a topic of interest, and you're taken to results that list both offline and online content. If you want to view only the digital (online) material, then take note of the Limit To box (on the right side of the results page), and click the link: Online items available.
Beyond that, there's little organization to the archive material. You can browse by contributing institution, but who really cares about that. You can also browse by collection title, which is only occasionally useful. There are no subject, keyword, format or theme headings that otherwise help organize the materials here, nor is there an Advanced Search page, a standard feature at most archive sites.
Also, don't go looking for family history databases also common to many other archives. There are no California vital record look-ups (births, deaths, etc) or online census records, though you may find ancestry information from just a general search. Seems a shame, but there you have it.
In contrast to the set-up at OAC, the archive material at Calisphere is beautifully organized into broad generational themes:
There are scores of sub-themes (e.g., Civil Rights under Social Reform), and hundreds of individual topic areas (Black Panthers). Calisphere is organized by the libraries of the University of California. This seems a very large collection, but again, nothing in the way of family history databases.
Lastly, there is also the official site of the California State Archives housed by the Secretary of State. Very little of their material is available online, but there are some interesting collections here:
California government history is covered by pages devoted to Governors, legislative histories, and even the working papers of the 1878-79 California Constitutional Convention.
There's a collection of the Robert F. Kennedy Assassination Investigation Records
Family history researchers will find some useful resources at this site, such as: