Fully conjugated polyazomethines (PAZs) exhibit large amounts of micropores among their many short branches in relatively planar structures based on their fully conjugated configuration. However, these fully conjugated structures cause insolubility and non‐processability of the PAZs because they may have strong stacking interactions due to their planar structures. In this paper, to overcome this problem, three short lengths of oligodimethylsiloxane (ODMS) grafts (x = 9, 18 and 39) were introduced by end‐capping reaction of aldehydes at the end of insoluble PAZs with amino‐terminated ODMSs as end‐capping agents. As a result, the insoluble PAZs became almost soluble (solubility > 95–100%) and their molecular weight could be estimated using gel permeation chromatography (Mw = 15.9 × 103 g mol−1). PAZ‐embedded mixed‐matrix membranes (MMMs) were fabricated using blend solutions containing only small amounts (1.0 wt%) of the solubilized PAZs with ODMS grafts and crosslinked polydimethylsiloxane (PDMS). The PAZs/PDMS MMMs showed ultrahigh permeability to oxygen which achieved 8600 barrer without a decrease of the original permselectivity of PDMS. The excellent oxygen permeability of the PAZs/PDMS MMMs may be caused by an effective increase of porosity based on the fully conjugated PAZ nanosheet. © 2022 Society of Industrial Chemistry.