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I have heard about that in the past, but i didnt have time to research it, so i dont know the answer to that.

Open ledger has a decentralized exchange. Although I still prefer to keep my crypto in my own wallets and only "transfer" what I intend to trade. I am a bit paranoid in that regard.
To answer your question...yes it's safe.

Of course, most of the coins should always be kept offline in your wallet, regardless of platform. What you use for trading should only be the amount you are willing to risk on it, and not your entire wealth.