How to find what application is using network port in linux?

netstat -nlp will give you an idea.

[root@os]# netstat -nlp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address               Foreign Address             State       PID/Program name
tcp        0      0 0.0.0.0:9921                0.0.0.0:*                   LISTEN      10982/ora_d000_orcl
tcp        0      0 10.10.10.10:1521           0.0.0.0:*                   LISTEN      12516/tnslsnr
tcp        0      0 0.0.0.0:22                  0.0.0.0:*                   LISTEN      6561/sshd
udp        0      0 127.0.0.1:1052              0.0.0.0:*                               10920/ora_pmon_orcl