Omeprazole is a proton-pump inhibitor that lowers stomach acid. Because acid affects how some drugs are absorbed, and because omeprazole influences certain liver enzymes, it can interact with medicines such as clopidogrel (where it may reduce the blood thinner's effect), some antifungals, HIV medicines, methotrexate, and certain anti-seizure drugs.
It is usually fine with everyday painkillers like paracetamol, and is often prescribed precisely to protect the stomach in people taking NSAIDs. Still, the safest approach is to give your pharmacist or doctor a complete list of everything you take, including supplements, before starting it.
If you are on clopidogrel after a heart procedure, do not start omeprazole without medical advice. Seek care for new chest pain, signs of bleeding, or symptoms that worsen despite treatment.