Open source is not a license. Open source literally just means that the source is openly available. It does not include the right for you to reuse or change any of the source.
That’s why most of the time, people are talking about “Free Open Source Software” (FOSS) when they think of openly licensed source code.
That’s why you can publish your project on e.g. Github (= open source) but if you don’t add a license statement, your work is still protected by an “all rights reserved copyright”. (= not free)
Anyhow, I would not necessarily deem a project OSS, just because the used language is readable by default. To me, OSS needs at least the developers intention to make it openly available.