There is no o-ring. These oil lines seal on a metal to metal flare. Try tightening it carefully. These fittings are famous for stripping the threads. If that happens MB has a repair fitting you can buy but it needs to be drilled and tapped. Are you sure that the fitting is leaking and it is not the hose? May be time to replace the oil lines.

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81 300D Turbo 190K sold
83 300sd 319K best $500 I ever spent-daily driverw/ 2 tank WVO set up