I have a 07 chrysler 300c 5.6ltr. The cel is on for fault po520. I've changed the oil, filter & sensor. I had the code cleared. But the light comes back on. Any ideas ?

That code indicates that the PCM isn't 'seeing' the correct engine oil pressure. This will need to be diagnosed hands-on by someone with a quality digital volt meter. There are many steps involved so this isn't recommended for the DIYer.
I wouldn't use this vehicle until the issue is resolved since if you're actually NOT getting proper engine lubrication, damage to the engine is imminent.

Proceeding carefully, check the oil pressure with a known good mechanical gauge. Check the engine for sludging. If there's sludge, consider Auto-RXing the engine; then, switch to full synthetic engine oil, which should eliminate sludge. If it's fine, and the engine runs okay, focus on the PCM and wiring.