With the UCo1-xOsxAl single crystals, we studied interplay between alloying and pressure effects on magnetism of the itinerant electron metamagnet UCoAl. The Os alloying for x ≥ 0.005 switches the UCoAl to ferromagnetism. For the UCo0.995Os0.005Al single crystal, even lowest applied pressure 0.11 GPa is sufficient to suppress ferromagnetism. A sharp metamagnetic transition for various pressures is observed in magnetic fields along the c axis of the crystal. Concentration magnetic phase diagram of the UCo1-xOsxAl, which includes the quantum phase transition from ferromagnetic to metamagnetic state, and P–T–H phase diagram of the UCo0.995Os0.005Al are determined using the data of magnetization measurements at various pressures and temperatures.