The main difference between these plans is that with a PPO, you can see a provider out-of-network, you don’t need a referral to see a specialist, and you are not required to have a primary care physician. With an HMO, you generally can’t leave your network to see a provider or specialist unless it’s a medical emergency or you require access to a specialist that is not in-network. HMOs generally have lower monthly premiums, coinsurance and copayments.