OS X EI Capitan problem with reinstalling

So quite awhile ago (few years actually) I was having fun with my new iMac that my dad bought for me. It was all going well but what I didn’t know was that the previous owners Apple ID was signed into it so when I went to reset it to clear storage, I tried putting in my “iPhone” Apple ID but it came up with a message saying “This item is temporarily unavailable. The iMac I am using is an older version but I am assuming it’s because the apple ID I am using is not connected with a Mac account. So is there anyway I can sign into my Mac and reinstall OS X? We no longer have contact with the previous owner so there is no way we are gonna know his Apple ID.