Pistorius, 27, is charged with premeditated murder in the shooting death of his girlfriend, Reeva Steenkamp, on Valentine's Day last year.
Pistorius fired through a locked toilet door in a bathroom in his home in the early hours of the day, fatally shooting Steenkamp.
The prosecution — led by Nel, who has been nicknamed "pitbull" by local media — has argued that Pistorius shot Steenkamp intentionally following an argument.
Pistorius maintains he shot the 29-year-old model accidentally, believing she was an intruder in his home.
Monday is Pistorius's fourth day being cross-examined by Nel, who has questioned the athlete about his ego and accused him of "adapting" his story.
Pistorius is the defence's second witness. The defence is expected to call more than a dozen witnesses.
Judge Thokozile Masipa is presiding over the case. She alone will determine Pistorius's fate as South Africa does not have trial by jury.
If convicted of premediatated murder, Pistorius faces 25 years to life in prison.- INTERACTIVE | The state vs. Pistorius: how the accounts differ
- ANALYSIS | Oscar Pistorius under the microscope
Timeline: Oscar Pistorius