Hi Vadim,
I cannot confirm that. To give you an idea, I invite you to enjoy this chapter of the manual about the LIMIT clause:
... or about the DUAL table:
Just to name a few very obvious differences.
You can of course try using the Open Source Edition with Oracle, but you won't get very far beyond a few simple JOINs, and whatever SQLDialect you choose from the Open Source Edition, it will suddenly not behave the way you expect. Is that risk worth the trouble?
And then, of course, there's the whole story of actual vendor-specific features (like PIVOT, CONNECT BY, INSERT .. RETURNING, stored procedures, LOBs, arrays etc.)
The relatively low overhead of commercial licensing will most certainly be much preferrable to the extra work of patching jOOQ and working around and documenting bad dialect fits all the time. Your jOOQ experience will be much better.
I hope this helps. If you have any additional questions, just let me know and I'll be very happy to assist you.
Best Regards,
Lukas