Is it possible to write 11g OCA with out any 10g certification .
Yes. I am assuming that you are referring to Oracle Database 11g Administrator Certified Associate. There is more than one 'OCA Certification'.
If you have no prior certification, the most common path to attain this certification is through the following two tests:
1Z0-051 - Oracle Database 11g: SQL Fundamentals I
1Z0-052 - Oracle Database 11g: Administration I
There are two other possibilities for the SQL test, but 1Z0-051 is most commonly chosen. You can get more details at the following URL: