Create config file for your custom command definitions in /etc/nagios-plugins/config/, name it whatever you wish, for example, my_comm_defs.cfg and in there you should create command definition. As an example how to do that view other files in that directory and copy paste one command definition to your my_comm_defs.cfg, and, of course, change the command_line variable to check_oracle, and take care of the path to your check_oracle plugin. It should look something like this:
command_line /usr/lib/nagios/plugins/check_oracle <OPTION> $ARG1$ $ARG2$ $ARGn$
Those $ARG#$ thingies are here just for example and those are the arguments which you would like to use.
Check the available options and arguments by starting the plugin check_oracle form terminal like this:
and see which one suits best for your needs. If you need only one then in command definition you will put only one $ARG1$
Then when creating a service which would check oracle, for check_command you should put the command_name of the command with arguments (in your case SID, I'll take the argument is my_SID) delimited by !
Hoep it solves your problem.