A pressure vessel is disclosed. The pressure vessel may include a boss, a liner, an O-ring, and a composite overwrap of shell. A boss may comprise a through aperture, a first groove encircling the through aperture, and a first engagement mechanism. A liner may comprise an interior surface, exterior surface, and a second engagement mechanism. The interior surface may define an interior cavity of the pressure vessel. The second engagement mechanism may mechanically engage the first engagement mechanism to secure the liner to the boss. An O-ring may be positioned within the first groove of the boss and abut the exterior surface of the liner. A composite overwrap may surround the liner and at least a portion of the boss.