The owb is model HF0406. The dimensions are 4x8x6 mm (ID, OD, width) and it is
here (the photo on the page is not the exact model).
Be aware that you need a tip that is OWB-ready. If you have a standard earlier bearing tip with a single diameter shaft, you won't be able to fit the owb as it has a smaller inner diameter than the regular bearings.
EDIT: For completeness, the regular bearings on the S8 tops are what yo-yoers call A-size: 5x10x4mm. The bearings for the Chico top are C-size: 1/8"x1/2"x3/16".