A refrigeration system comprising: at least one first compressor fluidly connected to a first suction line and a first discharge line; at least one second compressor fluidly connected to a second suction line and a second discharge line, the second suction line being fluidly connected to the first discharge line; a condenser operable to receive refrigerant from the at least one second compressor; a first evaporator operable to receive refrigerant from the condenser and discharge refrigerant to the first suction line; a second evaporator operable to receive refrigerant from the condenser and discharge refrigerant to the second suction line; and a first oil separator comprising a first inlet fluidly connected to the first discharge line, a first refrigerant outlet fluidly connected the second suction line, a first oil outlet fluidly connected to a first sump of the at least one first compressor, a first horizontal plane intersecting the first refrigerant outlet, wherein: the first oil separator is adapted to receive refrigerant and oil from the at least one first compressor through the first inlet and separate at least a portion of the oil from the refrigerant; a first portion of the first oil separator below the first horizontal plane is configured to collect separated oil; the first portion is sized to have a volume equal to a first compressor oil supply, the first compressor oil supply being greater than or equal to about 100% of a first compressor initial oil charge and less than or equal to about 250% of the first compressor initial oil charge; the at least one first compressor is configured to receive separated oil from the first oil outlet when an amount of separated oil in the first portion is less than or equal to the first compressor oil supply; and the at least one second compressor is configured to receive separated oil from the first refrigerant outlet when the amount of separated oil is greater than the first compressor oil supply. at least one first compressor fluidly connected to a first suction line and a first discharge line; at least one second compressor fluidly connected to a second suction line and a second discharge line, the second suction line being fluidly connected to the first discharge line; a condenser operable to receive refrigerant from the at least one second compressor; a first evaporator operable to receive refrigerant from the condenser and discharge refrigerant to the first suction line; a second evaporator operable to receive refrigerant from the condenser and discharge refrigerant to the second suction line; and a first oil separator comprising a first inlet fluidly connected to the first discharge line, a first refrigerant outlet fluidly connected the second suction line, a first oil outlet fluidly connected to a first sump of the at least one first compressor, a first horizontal plane intersecting the first refrigerant outlet, wherein: the first oil separator is adapted to receive refrigerant and oil from the at least one first compressor through the first inlet and separate at least a portion of the oil from the refrigerant; a first portion of the first oil separator below the first horizontal plane is configured to collect separated oil; the first portion is sized to have a volume equal to a first compressor oil supply, the first compressor oil supply being greater than or equal to about 100% of a first compressor initial oil charge and less than or equal to about 250% of the first compressor initial oil charge; the at least one first compressor is configured to receive separated oil from the first oil outlet when an amount of separated oil in the first portion is less than or equal to the first compressor oil supply; and the at least one second compressor is configured to receive separated oil from the first refrigerant outlet when the amount of separated oil is greater than the first compressor oil supply. the first oil separator is adapted to receive refrigerant and oil from the at least one first compressor through the first inlet and separate at least a portion of the oil from the refrigerant; a first portion of the first oil separator below the first horizontal plane is configured to collect separated oil; the first portion is sized to have a volume equal to a first compressor oil supply, the first compressor oil supply being greater than or equal to about 100% of a first compressor initial oil charge and less than or equal to about 250% of the first compressor initial oil charge; the at least one first compressor is configured to receive separated oil from the first oil outlet when an amount of separated oil in the first portion is less than or equal to the first compressor oil supply; and the at least one second compressor is configured to receive separated oil from the first refrigerant outlet when the amount of separated oil is greater than the first compressor oil supply.