A convenient strategy is proposed for improving the oil-absorbing performance of Calotropis gigantea fiber (CGF) by a simple immersion procedure that takes seconds (“instant”) in an ethanolic solution of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES). The fabricated CGF shows a roughed surface with the water contact angle of 130°, and with further combination of its intrinsic large lumen, this treated fiber PFOTES-CGF shows a remarkable improvement in oil absorbency, with an increase percentage of 52.2% for soybean oil, 22.0% for engine oil and 112.2% for kerosene, respectively. Owing to its hydrophobic–oleophilic property, the PFOTES-CGF can selectively collect oil spills from water with fast absorption kinetics, high absorption capacity and good recyclability, and also can be used for construction of a gravity-induced separation system for subsequent oil–water separation. This facile approach can be further applied to kapok fiber and proved to be very efficient in improving its oil-absorbing performance. Due to the abundance and biodegradability, upgrading this fiber via versatile methods may further explore its potential as an attractive alternative for oily wastewater treatment.