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The heating capacity and coefficient of performance (COP) of conventional air-source heat pumps decreases towards lower ambient temperatures. In addition, high discharge temperature at the compressor discharge might limit the operation of the heat pump at very low ambient temperatures. Oil injected into the compression chamber at the beginning of the compression process can absorb part of the heat generated during the compression process, which can result in significant reduction of the compressor discharge temperature. Discharge temperature decreases with increasing injected oil mass fraction, especially at low ambient temperatures. Therefore, oil injection allows the application of air-source heat pumps in regions with very low ambient temperatures in winter. Additionally, oil injection decreases the compressor power consumption by providing better sealing and lower friction during the compression process. Furthermore, if oil injection is combined with a regenerative heat exchanger and an oil cooler, the system performance of a vapor compression system can be improved significantly. The work presented in this paper shows the experimental results of a 5-ton (17.6 kW) R410A packaged heat pump which was retrofitted with an oil injected compressor, integrated indoor heat exchanger oil cooler and regenerator. The effect of different oil mass fractions on the system performance was investigated under standard AHRI 210/240 heating test conditions. According to the results, up to 8 % system COP improvement was observed compared to the baseline system for all ambient temperatures. 1