It must be cron , please check twice. 

@Saif : You are a developer.? I have checked the cron multiple times. But still issue exists....

Is there any plugin issue that can effect this.? 

Am not a developer But this issue is related with Cron only. 

Which cron command are you using ?
Earlier I faced this issue too but later used a different Cron Command and everything came to normal.

Ajay, if user just closes the browser without loging out, he stays "online" under "Members" sections as well as in chat for 30 minutes.

30 mins timelimit is set here: ow_system_plugins/base/bol/user_service.php:
method - deleteExpiredOnlineUsers

So after 30 minutes system checks whether user was active or not, if user stayed inactive for 30 minutes ( and he surely did, since he just closed the browser), CRON will "offline" this user.

If your users are stying "online" for more then 30 mins, then your Cron is not working.

I have watched out this issue. Issue exist till after 30 min, i have checked it for whole day but user not removed from list.

@Saif : I am using this command as cron
***** /home/site/public_html/ow_cron/run.php

What the other command it should be. if this is not a valid command, as This command shoot out the emails as well.
Pls help me...........