Yes, you can prepare well in 15 days if you spend about 6 hours per day.
First 7 Days
Read OCAJP book and Get some working knowledge in each exam objectives. Write programs and run it on your system.
Next 7 Days
Practice directly on any exam tools like MyExamCloud. Take OCAJP 8
mock exams and read explanations. After completing all mock exams, start writing programs again. Write program on each concept and try compiling with all possible options. The program directory created in your system must have chapter-wise folders for ease of review. Repeat this till you get good knowledge in exam chapters.
Review your weak areas and study well. Take final mock exam and try to achieve good score.
The key to achieve good score in real exam is solving as many mock exams as possible. Repeat your practice till you get 100% in all mock exams.