The simplest and most reliable first step is a letter. It requires nothing from him in advance, no approved list, no phone account, no prior arrangement. You write it, address it correctly, and it goes through the standard mailroom process and gets delivered at mail call.
OSCI refers to two well-known facilities depending on which state you are dealing with. Oregon State Correctional Institution in Salem, Oregon is one. Oshkosh Correctional Institution in Oshkosh, Wisconsin is the other. Knowing which one matters for getting the address right before anything goes out.
For Oregon, inmate information including current facility and DOC number is searchable at oregon.gov/doc. For Wisconsin, the offender search is available at doc.wi.gov. Both databases let you look up an inmate by name and confirm their current location and ID number, which you will need on any piece of mail.
Once you have his full legal name, inmate ID number, and the correct facility address, InmateAid can send the letter with their return address on the envelope. That keeps your personal home address private until you are ready to share it, which is useful whether this is a first contact or a reconnection after time apart.
A letter gives him time to read it privately and respond on his own terms without the pressure of a phone call requiring an immediate reaction. Once he responds and wants to establish phone contact, getting your number added to his approved call list is the next step, and that starts on his end once he has your information.
Start with the letter. It gets the ball rolling.