Where is the oil sending unit located?

Take a look at the attachments for the oil pressure sender. This is under the distributor so I am not sure it is the issue but I would suggest cleaning off the oil and then starting the engine after you top the oil off and see if you can locate the source.

Here is a guide that can help with this as well:

https://www.2carpros.com/articles/engine-is-leaking-oil

Let us know if you have other questions or what you find. Thanks