I'm 35 with 2 kids (P1 & K1), we have fully paid up our 4 room hdb flat with OA, now we are slowly building up our OA, thinking since now we do not have any use for OA, is it wise to transfer to SA?

I used to top up the maximum $7k to my SA account every year to get tax relief. just be aware that if you transfer money from your OA to SA, the amount in which you can top up to your RSTU will be reduced accordingly

I only did it for my parent when they are confirmed not going property. The upfront CPF/cash is usually the limiting factor of what house yo can get