Oracle offers variety of Java Certifications for all Java Editions (Java SE, Java EE and Java ME). Oracle Certified Associate, Java SE 7 Programmer certification provides basic knowledge required to program Java code.

What exam I need to pass to achieve OCA for Java SE 7 Certification?

You need to complete 1Z0-803 Java SE 7 Programmer I Exam to achieve this certification.

What kind of knowledge I can achieve after completing this certification?

You can gain basic knowledge of Java data types, strings, arrays, loop constructs, methods and encapsulation, inheritance and exceptions

Do I need any prior certification to take this exam?

If u have interest to do this certification please watch out preparation article at http://www.epractizelabs.com/blog2/?p=413