The boiling point of oxygen is approximately $90\text{ K}$. When liquid oxygen at $50\text{ K}$ is heated, its temperature will rise until it reaches its boiling point ($90\text{ K}$). During the phase transition from liquid to gas at $90\text{ K}$, the temperature remains constant as the heat supplied is used as latent heat of vaporization. Once all the liquid has converted into gas, the temperature of the gaseous oxygen will rise again until it reaches $300\text{ K}$. Thus, the temperature-time graph will have an initial positive slope, a horizontal segment (plateau) during boiling, and another positive slope.