Ho³⁺/Yb³⁺: BaMoO₄ phosphors with different concentrations were fabricated by a gel combustion method. The upconversion (UC) luminescence, intrinsic optical bistability, and the corresponding mechanisms were reported for the present system. The optical thermometric properties based on red (⁵F₅→⁵I₈) and green (⁵F₄/⁵S₂→⁵I₈) emissions were studied. The sensing sensitivities could be tuned by manipulating the cooperative energy transfer process. The highest absolute sensitivity was 99 × 10 ⁴ K ¹ at 573 K, which is larger than that of many previous UC materials.