Oxalis' have a winter dormant period that must be respected. Starting in September, stop fertilizing and gradually reduce the frequency of watering over the next month. The leaves will gradually die.
By mid October you should stop watering altogether and move your Oxalis to a cool, dark place where you can ignore it until February.
In February, you will see some signs of life. That is the signal that you can resume normal watering and fertilizing and move it to a sunny windowsill. Continue this routine until next September. It does best when moderately potbound.