I'm in my mid-30s and getting married next year. I have 30K in my OA currently, would it be better to pay a lump sum for my HDB loan first or would you advice transferring some to SA or keep in OA?

It depends on whether you have other income sources or savings that allows you the ability not to touch your OA. If you can afford to pay the down-payment from other sources, then you can consider transferring to SA.

For my own case, I topped up the down-payment with cash as I didn't want my OA to be wiped out.

But if you do not have the means to make down-payment in other ways, using OA first is a better option.

I would keep in OA as you still need the $ for HDB