Both come from Camellia sinensis, but oxidation separates them entirely. Green tea is 0% oxidized — leaves are fixed immediately after picking by heat (pan-fired or steamed). Oolong sits at 10–80% oxidation, giving it a spectrum from almost-green to almost-black.
Catechin content: unoxidized green tea retains more EGCG and other catechins. Oolong's partial oxidation converts some catechins into theaflavins and thearubigins — different antioxidants with their own properties, particularly relevant for cardiovascular and metabolic health.
Caffeine: similar per gram of dry leaf (25–40mg per 200ml cup). However, oolong is typically brewed in higher leaf-to-water ratios gongfu-style, so a full gongfu session (5–8 steeps) delivers more total caffeine over 45–60 minutes compared to a single green tea brew.
Flavor profiles: green tea ranges from vegetal and grassy (Japanese sencha, gyokuro) to nutty and sweet (Chinese Dragon Well, Bi Luo Chun). Oolong ranges from light floral and creamy (lightly oxidized Tie Guan Yin, high-mountain Taiwanese) to roasted, mineral, and complex (Da Hong Pao, Dong Ding).
Stomach sensitivity: oolong is significantly gentler on an empty stomach. Green tea's high catechin and tannin content can cause nausea or discomfort when drunk without food. If you experience this with green tea, switch to a medium-oxidized oolong as your baseline morning tea.
Practical decision guide: choose green tea when you want maximum antioxidant density, a metabolic boost, or a sharp morning activation. Choose oolong when you need a sustained midday state, want flavor complexity across multiple steeps, or drink tea with or after meals.
Entry-level recommendations: for green tea, start with Chinese Dragon Well (Long Jing) — clean, nutty, forgiving. For oolong, start with a lightly oxidized Tie Guan Yin or a Taiwanese Four Seasons — both are affordable, consistent, and accessible to developing palates.