update OID with a particular value in RedHat

What do you mean update?
First you have to have a write community with which you are setting.
The other the OID has to be read/write capable.

You can setup an OID within snmpd.conf that will trigger a script that does what you want and returning the value to the snmpwalk process.

Look at the other examples within /etc/snmp/snmpd.conf

that define how to get a response for a specific OID.

I have created a script to analyze the number of invalid request from apache log within last 5 min. And I want to put this value into OID such that it can be displayed on MRTG. There existing a MRTG graph that can do similar kind of thing but I have no idea how to create it. Any idea ?

I only need to setup syslog-ng server, correct ?

Do I need to configure anything on apache server, say the Apache configuration file - httpd.conf under /etc/httpd/conf.d ?

Tks

Apache needs to be instructed to log error logs and transfer log to syslog.
IMHO, you can do with rsyslog most anything,  what is it that attracts you syslog-ng.

All the information to answer your question has been posted.

Apache uses syslog facility local6
Rsyslog is configured to
Local6.* @central_syslog_server
Local6.* also_write_to_local_file
Local6.* can also pass this data to a program that will generate database entries, email notices, whatever you can think of. Including generate an SNMP trap to a monitor application that a particular preconfigured event/s occurred.

Current operation is using syslog-ng. Hence, I can change it.
Sorry for keeping asking for the question, but I really have no idea where I can configure it....

"Apache uses syslog facility local6
 Rsyslog is configured to...."
 
Do you mean configure syslog on Apache to forward it to my centralized Syslog_NG ? Tks

Apache httpd.conf configures Apache to log locally to the local syslog/rsyslog.
The local syslog is then configured to forward what it receives to your central syslog server.

It seems this question us devolving to a non related of your other questions.
Your snmpd.conf needs to reference a script that will either crunch the data when polled, or pull data from a flat file, while there is another process that crunches data.