How much should I transfer from my OA to SA to save on my income tax. And also I heard it is the best time to do this in January every year?

I would recommend to do OA-SA transfers only if you will not need those funds for education or housing in the near future since it's irreversible. If you want to qualify for income tax relief you should be considering RSTU to your SA or VC to your MA instead.

Unfortunately, transfering from OA to SA does not have any impact on your income tax. You will need to top up your SA using cash. The maximum CPF Cash Top-up Relief per Year of Assessment (YA) is $14,000 (maximum $7,000 for self, and maximum $7,000 for family members).

And yes, January is the best time to do it so that you can maximise the interest of 4% you can earn for the year.