Dear Customer,
The fitting you have is for OD16 mm hard tube.
So you need two different size Orings.
For the inner two orings, the model is BP-ML16-O10-BK.
Also, you can use the tool BPTA-ORFT16 to put the Oring inside easier.
https://shop.bitspower.com/index.php?route=product/category&path=59_128_138
The other Oring is for G1/4 thread. Here is the link.
https://shop.bitspower.com/index.php?route=product/product&path=59_128_374&product_id=410