It's not necessarily a big problem. How much oil is there? Clean out the filterbox, replace the filter and then get a good mechanic to check the PCV valve. This is a one way valve to release pressure from the bottom end of the engine. If it sticks, oil can 'burp' up into the air filter box. This is not a major repair and is reasonably common.
If your looking at more serious repairs the attached website is a guide to what the more serious options could be.
https://itstillruns.com/causes-engine-oil-blowby-air-filter-box-12246178.html
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