The Cuba international midfielder spent seven years with Norwich City, including a spell in the Premier League.
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Hernandez has initially penned a deal to January with the Addicks and said: "I’m really happy to be here.
"I’ve known the coach for a long time and I’ve played against his teams. He’s a very good coach who knows what he wants from his players.
"As soon as I had a conversation with Nathan, I knew straight away that it was something I wanted to be a part of."
Boss Nathan Jones added: "Onel brings real good Championship experience.
"We’ve had a look at him as he’s been training with us for a few weeks and he’s got wonderful athleticism and real one-v-one quality.
"We felt that we just wanted to add in that department.