Just thought I'd help out by giving some pointers and insight with regards to the SIA cadet pilot interview as I recently got through.
As you may already know, first interviews are conducted in 2 sessions every Friday. Second interviews, 1 full-day session every Wednesday. From what I could see, every Friday, there are about ~40 first interviewees (give or take). About 7 - 8 will make it to the second interview.
The second interview is variable. There have been days when all made it through or none at all. But from what I could gather, the usual is 4 ~ 5 will make it through.
That's a whopping total of 90% attrition rate.
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First interview.
There will be one HR personnel and one Captain. This is where it boils down to how lucky or how good you are. Some people get interviewed by Management Pilots, and these people tend to ask really difficult questions. Some, by line Captains who are generally laid-back and want to assess your passion to fly. Be prepared and read up more.
The general rule of thumb is: If you do not know the answer, tell the truth and say you do not know. Do not try to lie your way through. If they press on for the answer, then try to answer as logically as you can. They are interested to see your thought pattern.
Questions are not fixed and I believe they are tailored to the individual's background and qualifications. If you're a Business/Arts grad, be prepared to be asked questions pertaining to the basic engineering (what are the 4 engine strokes, how does the fuel system work) or basic flight (gas turbines, Bernoulli's effect in flight).
Management pilots like to ask business based questions as they want an "all-rounder". Questions may be in the form of: "Why does SIA invest in so many different types of aircrafts?", "Why do we fly only certain routes and not others?". There are technical questions too, so be prepared.
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Second interview.
The prerequisite computer test by ComPsy. Most of us found it difficult and felt like we failed it after we finished. It'll take around 2.5 hours. The full details of the test is in Pg78 of this thread. Most important thing here is: Get a good night's sleep the night before the test. It is mentally draining and you need to be alert.
Break for lunch.
Interview starts with a panel of 4 Captains and 1 psychologist. Same case here, the questions are tailored for you. Nothing is fixed, but be prepared to be asked similar questions that you couldn't answer in the first interview. I believe the first interviewer recorded the questions that you couldn't answer. Do not just read up on that specific question, dive deeper. Say if you couldn't answer what are the 4 engine strokes, read up on that and go further to include the fuel system, transmission system etc etc. They want to see that you are willing to learn.
Here, they are also interested in your background. Why you did certain things and why you didn't. If there's something I learnt from the many interviews that I've gone through, don't just highlight the positive and try to make yourself shine. You need to show your humanistic side too. Do not be afraid to say: "It was a mistake on my part.. I regret not having pursued that when I was.. If I could turn back time, I would.." They want to see that you know your weaknesses, your mistakes and are willing to change that if given a chance.
As for the tea party, its a relatively informal affair at the training centre lounge. Small table with coffee, plain water, cakes and puffs. You can ask the 4 Captains about their experiences, their take on flying, their advice on certain matters. Be nice and don't try to dominate the entire conversation. Don't be too quiet either.
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With regards to the question of age, for Singaporeans, at 25, you may apply. But SIA will only give you the contract once they have sought clearance from the government (RSAF). At 24, you're a little too young in their eyes. If you get rejected through their silkroad system, apply again at 25. It's unfair, yes, cos the foreigners get to go through at 20, 21. But it's just the way our country's system works. Accept it and don't try to fight it.
As to "O" levels, even if you retake one paper that you didnt do well for (say you got D7 for Physics and wish to retake only Physics right now), it will not make a difference. From what the HR told us, they want to see you take all the papers in ONE sitting. This is to judge your ability to get through the 14 ATPL ground papers in 6 months. Reason being "O" levels is a better gauge is because at the University level, you can OTOT (own time, own target) pass the specific module, even if its 10x harder than "O" levels.
Turn your positive experiences and word it in such a way that it benefits SIA. If you did well in sports, say your athletic abilities stem from your constant desire to excel and it is this quality that you believe SIA needs from its pilots. If you're good academically, mention your hard-working traits and how SIA would benefit from this. In short, all your qualities should BENEFIT SIA, and not just yourself.
All the best!