To Encrypt or Decrypt we will need a set of keys.
We will use Public Key to Encrypt the Data and Private Key to Decrypt the Data.
I took this image from https://comodosslstore.com/blog/public-key-and-private-key-pair-how-it-works.html

We can now visit https://pgpkeygen.com/ to generate pair of Keys. Or below method can be followed too. However pgpkeygen is widely used.
I used below settings.
NOTE : Copy the password in a notepad before you forget it. We will need this password while configuring SFTP Connection in OIC.

This gave me two files like below :-

Now create a SFTP Connection and configure like below :-



Test and Save the Connection.
The Integration looks like below :-


Rest Sample Payload for inline methods.
{"firstName" : "x",
"lastName" : "Y"
}
I have created the below simple CSV file to use in File Write Operation:-





Once activated and Tested got a successful response.

Thanks for reading.
Tips:
In next Step Try using password free Authentication for SFTP Server.
Use DVMs to map the filename patterns and paths.