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ICE BofA US Corporate Index Option-Adjusted Spread. The yield premium that investment-grade (BBB-rated and above) corporate bonds trade at over Treasuries.

Why it matters: Investment-grade spreads are a more conservative, less noisy complement to high-yield spreads. They move less violently but track the same underlying story: risk appetite, credit stress, and the cost of capital for healthy companies. When *even IG* spreads start blowing out, it's a serious signal — tends to only happen in genuine crises.

How to read it: Compare directly to BAMLH0A0HYM2 (HY spread). The HY-IG ratio is more stable than either alone and tells you relative stress between quality tiers. Historical IG spread range is roughly 80-200 bps. Above 300 bps is crisis territory.

Caveats: As with HY OAS, it's option-adjusted and embeds some index-composition drift over time. Not a fast-moving indicator in normal conditions.