The effect of an annular, piston bowl-rim cavity on in-cylinder and engine-out soot emissions is measured in a heavy-duty, optically accessible, single-cylinder diesel engine using in-cylinder soot diagnostics and exhaust smoke emission measurements. The baseline piston configuration consists of a right-cylindrical bowl, while the cavity-piston configuration features an additional annular cavity that is located below the piston bowl-rim and connected to the main-combustion chamber through a thin annular passage, accounting for a 3% increase in the clearance volume, resulting in a reduction in geometric compression ratio (CR) from 11.22 to 10.91. Experiments using the cavity-piston configuration showed a significant reduction of engine-out smoke ranging from 20-60% over a range of engine loads. To understand the effect of geometric CR on smoke emissions, two additional piston configurations with lower CRs (10.75 and 10.32) achieved by removing portions of the piston bowl-wall are also studied. Engine-out smoke generally decreased with decreasing CR, but the cavity-piston configuration shows an additional soot reduction relative to its CR. One hypothesis explored is that amplified late-cycle mixing and oxidation associated with flows into and out of the cavity are responsible for the additional soot reduction. Comparative analysis of apparent heat release rates indicate a measurable increase in the late-cycle oxidation immediately following the peak in-cylinder pressure for the cavity piston configuration. This is consistent with the timing of when the cavity contents are expected to begin discharging into the piston bowl, supporting the enhanced late-cycle mixing and oxidation hypothesis. Optical diagnostics, including quantitative measurements of soot optical density KL (diffuse back-illuminated imaging, DBI) and soot temperature provide spatially and temporally resolved information about the effect of mixing on in-cylinder soot distributions near the piston bowl-rim (cavity passage). Spatially-averaged, late-cycle soot-KL trends agree with engine-out smoke data, suggesting that the in-cylinder soot-KL within the DBI field of view is representative of soot for the entire combustion chamber, at least late in the cycle. Soot-temperature distribution maps calculated from the soot-KL values and the absolute soot-incandescence intensity indicate higher soot temperature (exceeding the baseline piston by 100-200 K) near the cavity exit passage, which is also consistent with the enhanced late-cycle mixing and oxidation hypothesis.