Three things that matter the most in the oral tests -

1. Language and presentation
2. Thought process
3. Confidence

The language that one is  supposedly to use in SSB is English. Now first thing one  should do is work upon one’s English speaking skills. Increase your vocabulary. Enrich your English. Work upon the accent and tone. Work upon presentation.

1. Start reading newspapers daily. Mark the words which you don't know, note them down with their meaning. Now when you speak try including those words in.
2. Try reading the newspaper aloud and while reading it videotape yourself using your phone. Check that video to see what was your body language at that time, how you were modulating your voice, how you were pronouncing stuff, how your accent was and keep on improving it on a daily basis.

Now second thing/reason coz of which people will listen to you is your thoughts. Work on your thought process.

1. Take up a random topic each day prepare a 4–5 minute speech on it. Think about the topic analytically, search for stats related to the topic. Then give that speech standing in front of the mirror and rate yourself, your content, and the way you spoke it.
2. Read good books that improve your thinking ability.
3. Start perceiving things differently, develop views about stuff you read or see express those views whenever you have a chance.

Third and the most important thing is how confident you are while speaking whatever you are speaking because if you aren't confident while speaking all your content and good English stuff would be of no use.

1. Grab each chance that you get to do public speaking. On your friends birthday you can give a short speech in front of the people gathered there, if you learn a new chapter you can explain it to your mom, dad or friend, take part in each debate, poem recitation and all that sort of competitions.
2. Write about your hardwork how you are working on your POE this will give you self confidence.
3. Get as much exposure as possible, interact with as many people as possible, this too will give you confidence that you can talk to anyone and everyone and you can express your ideas in front of him/her.