Depends on exactly what you mean.
Let's say that I'm employing you to write a trading platform. You research the field, find some OSS that can serve as a base to work from and modify it to my specs. I own the modified code and can report you to the DA if you take it and try to sell it somewhere else.
Now if instead, you just take the original OSS you found, and the knowledge you gained working from me and go somewhere else to build a trading platform based on that original source, I have no case against you (assuming you aren't using my trade secrets) since you haven't taken anything I own.
Does that help?