Complex oxides hosting 5d electrons present a variety of exotic phases arising from spin-orbital interactions and electronic correlation. In the Mott insulator Ba2NaOsO6 (BNOO), a canted antiferromagnet with multipolar interactions, strong electronic correlation together with Jahn-Teller lattice activity pave the way for bridging polarons and SO coupling, distinct quantum effects that play a critical role in charge transport and spin-orbitronics. Polarons are quasiparticles originating from strong electron-phonon interaction and are ubiquitous in polarizable materials, especially in 3d transition metal oxides. Despite the more spatially delocalized nature of 5d electrons, we demonstrate the formation of Jahn-Teller spin-orbital bipolarons in electron doped Ba2Na1−xCaxOsO6 by combining ab-initio calculations with nuclear magnetic resonance and muon spin rotation measurements. The polaronic charge trapping process converts the Os 5d1 spin-orbital Jeff = 3/2 levels, characteristic of pristine BNOO, into a 5d2 Jeff = 2 manifold, leading to the coexistence of different J-effective states in a single-phase material. Moreover, we suggest that polaron formation creates robust in-gap states that prevent the transition to a metal phase even at ultrahigh doping, thus preserving the Mott gap across the entire doping range from d1 BNOO to d2 Ba2CaOsO6.