Black-glazed porcelain holds a significant place in the history of Chinese ceramics, with Song dynasty Jian kiln oil spot glazes being highly esteemed for their distinctive colors and patterns. To elucidate the compositional characteristics and underlying causes of the varying colors in oil spot glazes, we employed energy dispersive X-ray fluorescence (EDXRF), colorimetry, scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), micro-focused Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) to analyze the composition, microstructure, and firing processes of these glazes. The results indicate that the Si/Al ratio of Song dynasty Jian kiln oil spot glazes ranges from 6.77 to 11.15, distinct from other Jian ware glazes. These oil spot glazes are characterized by high silicon, high potassium, low calcium, low iron, and low titanium content, suggesting a unique formulation. There is a significant difference in elemental distribution inside and outside the crystalline spots, with calcium, iron, titanium, and phosphorus being enriched within them, playing a crucial role in their formation. No significant differences were observed in the chemical composition of oil spot glazes with different colors, indicating that composition is not the primary determinant of coloration. Instead, analysis of iron ion valence states in the glaze and body color suggests that the firing atmosphere is a key factor influencing the type and size of precipitated crystals. When the Fe2+/Fe3+ ratio is high, the body color tends to be darker, indicating a predominantly reducing atmosphere. The silver-white spots are caused by strong light reflection from aggregated ε-Fe2O3 nanocrystals or α-Fe2O3 crystalline films. Conversely, when the Fe2+/Fe3+ ratio is low, the body color tends to be reddish, indicating an oxidizing atmosphere. The brownish-yellow and reddish-brown spots primarily result from the chemical color of 5～10 μm snowflake-shaped ε-Fe2O3 and α-Fe2O3 crystals. The coupling effect between chemical and structural colors can modulate the glazes coloration. Yellow-green spots with a blue tint arise mainly from Rayleigh scattering of 100～200 nm ε-Fe2O3 particles coupled with chemical color, while silver-white spots with a reddish-brown tint result from total reflection by α-Fe2O3 crystalline films coupled with chemical color. This study elucidates the composition and structural characteristics of Song dynasty Jian kiln oil spot glazes, revealing their diverse coloration mechanisms. These findings provide important insights into the technological aspects of Jian black-glazed porcelain and have implications for the innovation and development of iron-based crystalline glazes.