And what is OOAK?! Both great questions. One of a kind (or OOAK) jewelry is an exclusive level of handmade and design. Similar to custom made pieces but you haven't commissioned it or worked with the designer.
People who are drawn to this type of product appreciate that handmade skills take time to hone and also like having something unique. It may be that they admire a particular artist or genre.
All jewelry has some handmade elements to them, it's just how much of the process is now mass-produced and is the design original or conceptual.
For example, a jeweler may fabricate the entire piece. Fabrication is where the piece is built up from metal sheet and wire etc. In this case, the warehouse may employ many jewelers producing the same piece or type of piece over and over. In this case, the entire piece may be handmade but not a unique design. Chances are it's a bit of both, the claw setting or band has been mass cast, the jeweler resizes to fit the stone for example. Though stones can be machine cut also to a set size, making the process easier still.
Then there's a mass cast of the entire piece and the finishing is done by hand. In this process, hundreds of the same thing have been formed by pushing the wax into a mold and cast many at once in large facilities. Cost-Effective use of volumes.
A mid-level process is where molds and casting are used but a limited number of each range is produced.
And then there's OOAK. The artist may have produced many prototypes and samples before arriving at the final piece. Typically they will have a style, look or statement about the work that sets them apart. This usually comes from somewhere personal, the artist may share their story or vision.
What you're paying for then is their skill, vision, and passion. In my case, you are also paying for quality precious metals and stones.
No one system is right or wrong, or better than the other, it's simply what you see as being of value to you.