The Okara-Tempe Hypothesis (OTH) proposes that early tempe-making in Java most likely began as a waste-to-food practice: okara (tofu residue) from tofu production was widely available and inexpensive, and under tropical conditions it could be naturally colonized by Rhizopus spp. during leaf-wrapped storage, producing a cohesive fermented product that people learned to reproduce and later "upgraded" to whole soybeans when economically feasible. This hypothesis synthesizes evidence from multiple independent sources: (1) Indonesian food historians explicitly linking tempe origins to tofu-factory waste colonized by mold, (2) international fermentation scholars suggesting okara-first with subsequent shift to whole soybeans, (3) historical documentation showing tempe was culturally established by the early 19th century (Serat Centhini, 1814), (4) pattern evidence from other fermented by-products (tempe bongkrek, tempe gembus), and (5) modern biological feasibility studies confirming okara can be fermented into tempe-like products. The document employs transparent methodology: each claim is analyzed independently with strength ratings, gaps are acknowledged explicitly, falsifiable predictions are provided (what evidence would disprove the hypothesis), and alternative explanations are considered. Confidence is estimated at 70-75% that okara fermentation was the primary pathway for tempe's origin, with uncertainty acknowledged regarding exact dates, geography, and whether multiple simultaneous origins existed. This work contributes to Indonesian food history by providing the first comprehensive synthesis of scattered hints about tempe's origins, demonstrates waste-to-value innovation as a historical mechanism, and illustrates how traditional knowledge systems develop through empirical observation without formal scientific training. Keywords: tempe, tempeh, okara, Rhizopus oligosporus, food history, fermentation, Indonesia, waste upcycling, traditional knowledge