Optical fibers highly-doped with aluminum oxide and holmium ions for a use in fiber lasers were prepared and tested in a laser arrangement. The fibers were prepared using modified chemical vapor deposition method in combination with nanoparticle-doping method. Relations between fibers composition and their laser parameters were studied within a series of various Al2O3 and Ho3+ concentrations. Fibers with fluorescence lifetime above 1.5 ms were prepared and lasers with slope efficiency up to 86% and laser threshold below 50 mW were constructed. Molar ratio Al/Ho equals to at least 50 was found to be the key factor for fibers with such excellent parameters. Thanks to high concentrations of Al2O3 (up to 10 mol.%), enabled by nanoparticle doping, we achieved high performance in a wide range of Ho3+ concentrations (1000–6000 ppm). The results will be used in the designing of highly-efficient high-power cladding-pumped holmium fiber lasers.