I see that indeed the OneDEX website is unavailable. The project seems related to another project Onechain. As there wasn't any further information to find about One DEX, I went looking for what I could find about Onechain, here are the red flags:
Github: Only 80 contributions in the past year, but no activity since Sept 2018.
Twitter: Last Tweet in Dec 2018
Medium: last article in Dec 2018
One chain website: very bad loading and horrible interface. A lot of high performance claims made, but no technical details given (as the whitepaper is also unavailable; broken link)
No wonder that someone with the profile name ODEX is going to post a positive review about this project, smells a lot like conflict of interest and certainly not unbiased.
Website under maintenance so the service is unavailable, according to dappradar this EOS dapp has had 18 users in the last 7 days, 65 transactions and 30 EOS transaction volume, so nothing to write home about, also link to whitepaper on onechain website (apparently onechain developed One DEX) is broken so whitepaper unavailable. I stop my research here because I have already seen enough red flags.
If someone can add some more relevant and credible information that adds to its credibility I might adjust my rating slightly, for now I will assume the worst.