Two 2022 studies, starting with “Increasing bound antioxidant compounds through their reaction with soluble phenolic compounds”:

“Wheat, oat, rye, and rice bran samples were reacted with different concentrations of beverages (green tea infusion, black tea infusion, espresso, and red wine) rich in various soluble phenolic compounds.

• Green tea infusion was found to be the most effective beverage.
• pH rather than time and temperature had significant effects on the reaction.
• Neutral or slightly alkaline conditions (pH 7.0-7.9) and mild temperature (at about 50 °C) were found to be optimum to increase antioxidant capacity of cereal bran samples.
• Total antioxidant capacity of oat bran treated with green tea infusion at optimum conditions (53.3 °C, pH 7.4, 60.0 min) reached 226.42±0.88 mmol.
• Free amino groups in cereal bran were also found to decrease 32–95% after treatment.”

https://onlinelibrary.wiley.com/doi/10.1002/jsfa.12017 “Optimization of reaction conditions for the design of cereal based dietary fibers with high antioxidant capacity” (not freely available)

Hadn’t thought about purposely combining oats with green tea before. I eat whole oats, though, not oat bran.

The same coauthors earlier used an in vitro digestion procedure to investigate combinations of 20 foods purchased from local markets:

“Individual antioxidant capacity of a single compound is not adequate to assess antioxidant potential of food or human plasma. Compounds always present as natural mixtures, and may possess similar, overlapping, or different but complementary effects.

Certain types of foods co-existing in daily diet were investigated in terms of their combined total antioxidant capacity (TAC) determined by the QUENCHER method, which allows physiological evaluation without any extraction procedure. Hydroxyl radical scavenging capacity was also determined in bioaccessible fractions of foods.

Interaction types were determined at each step:

• Synergism refers to a greater overall effect in the combination of two samples compared to simple addition of their individual effects, which means that TAC_measured is greater (p < 0.05) than TAC_estimated.
• The phenomenon in which a lower (p < 0.05) net interactive effect than the sum of their individual effects (TAC_measured < TAC_estimated), is known as antagonism.
• Additive interaction occurs when a net interactive antioxidant effect is as same (p > 0.05) as the sum of individual effects.

• Seeds and nuts interacted antagonistically with other foods due to the pro-oxidant potential of transition metals on lipid rich system.
• Protein-phenol interactions masking TACs of phenol-rich foods before digestion could stabilize and regenerate phenolic compounds under gastrointestinal digestion conditions, providing a synergistic interaction.
• Intestinal conditions promoting reaction between antioxidant compounds and radicals resulted in increases in TACs of foods.
• Enzymatic colonic digestion caused significant increases in TACs of certain foods.

These findings provide a basis to increase antioxidant activity in daily diet and new food formulations.”

https://www.sciencedirect.com/science/article/pii/S2665927122000351 “Effect of food combinations and their co-digestion on total antioxidant capacity under simulated gastrointestinal conditions”

View this second study as representative or hypothesis-generating, but not specifically definitive. No research group will use its resources to investigate even the 190 pairwise combinations of 20 foods, much less all 616,645 combinations.

Also, since food is digested all in the same place and time, contexts for each combination’s synergistic, antagonistic, or additive activities may be influenced by other combinations’ results. See the second study of Dietary contexts matter for a similar investigation.