O level private preparation :
For most of the things you can prepare for yourself.
Buy the most voluminous book on the subject from the library. This preferably should be written by some western. Don't buy Indian writers book. The difference is that Indians focus too much on rot memorization just like Bangalies. And their books reflect that. No explanation, nothing to understand, your teacher is supposed to explain you the topic and the whole book is full of questions and exercises so that you get a good grade. Skip those books.
The book you buy should have a lot of white space on the left and right column, and explain everything in details with a lot of pictures and graphs. Buy this one.
Next go through it from start to end. Like a story book. This is your 1st read. Might take 2 weeks.
When done, go through it a second time, this time focus on the math and try to understand everything. Might take 2 months.
Last go through it a third time. This time only focus on things you couldn't really grok previously
For science students. One thing you need most is Science practicals. For this you need to get admitted into one of your nearby coaching centers.