I'm in Army Reserve and am about to apply for OCS. Can I pick active duty or do I have to stay in a Reserve officer?

ONCE YOU HAVE BECOME A 2lt. its much easier to go active. but to answer the question, no, you cant just go active. you must apply after you graduate.

As far as I can contribute, you will remain on active duty in the OTRA (Other than Active Duty). You will serve on active duty in the Army as any other RA competitive category, but you belong to the Reserves. After active duty, you remain in the Reserves till your retirement. On active duty, you will be managed by the PERSCOM in Alexandria and post active duty by the Reserves PERSCOM in St. Louis. This could have changed, so verify it please.