A 2.6-MV field-distortion oil switch was developed and tested. Engineering design theories of this switch, including electrode areas, oil anode-cathode (A-K) gaps, insulator-surface electric field, and A-K gaps of the pre-gas switch, were described in detail. Structure designs of the switch were presented. Electric-field distributions of the oil A-K gap and insulator surface were analyzed and discussed. Also, switch breakdown processes were estimated. Finally, performances of this switch were tested on the “JianGuang-I” facility (2.3MV/350ns). Preliminary experiment results show that with a 250kV trigger voltage, the switch-breakdown time delay is about 132ns and its jitter (the standard deviation) is less than 4.4ns.