Olmo started on the bench and came on at the break, with Barca 1-0 down.
In the end, the new signing struck the winner as Barca triumped 2-1 in Vallecas.
"I have been waiting for many days, and ever since I arrived I have been waiting for my debut. What better way than to win here where we haven't won in many years? It is unprecedented," Olmo told DAZN.
"I felt very good on the field, also physically. Wherever the coach puts me, I will do my best. Today (yesterday) I played behind (Robert) Lewandowski, but I can play on both sides."
The 26-year-old took the opportunity to celebrate his goal together with Lamine Yamal, 17.
"I am a player who always wants to score goals and create opportunities. Lamine found me very well and I was able to score. I am very happy.
"We are prepared for anything, we never give up. We push and go for it until the 100th minute like today, until the last moment. I always celebrate like this when I score. I spoke to Lamine and we said if I scored we would do it together, and that's what happened. I dedicate it to my family as well."