PPO medical plans offer direct in-network access to a large variety of healthcare providers in most geographic areas. On a PPO plan, usually you can call around and get an appointment with a provider who has availability within 7 - 10 days or sooner for urgent health issues. You can take matters in to your own hands to find a provider who will see you.
HMO plans are more restrictive and require members to see your designated primary care primary care physician and get referrals for specialists, diagnostic tests, physical therapists, etc. As a consumer protection, the State of California has legal guidelines that HMO plans must follow. If your HMO is not providing timely access, this is a helpful document to reference.