Clear HDB loan first or use the OA to transfer to SA to beef FRS? ?

Yes, it is a good idea to transfer OA to SA. You will get a 1.4% positive roll on your money. CPF has its own calculator to tell u how much more interest u earn by transferring OA to SA. HDB loan is just 0.1% more so the estimation will not be too far off.

https://www.cpf.gov.sg/eSvc/Web/Schemes/OAToSAT...

But if you value the peace of mind in paying down your loan faster, just go ahead and use your OA.

Have you considered topping up your SA using cash?

You will receive tax relief for the topped up amount. Eg, your taxable income is $40k, for every dollar u earn above 40k is taxed at 7%. Therefore, if u topped up $1000 into your special account, u get $1000 tax relief, u saved 7% of $1000 = $70 tax money. Effectively have gotten 7% return on your money, in additional u will earn 4%% interest yearly guaranteed. Tax relief through this way is capped at $7k.

The monthly payout (CPF Life) will payout for the rest of your life. It will not be depleted.

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Take the FRS out of the equation first, because regardless, you're still obligated to pay off the HDB loan.

Then consider this:

Although SA offers 4% interest rate as compared to the 2.6% interest accruing on your loan of OA may make it sound like a good choice to transfer OA to SA, it's not entirely true.

Note that your 2.6% interest accruing on your outstanding loan of 170k would mean that it's a loan interest of $4420 annually.

Even if you were to transfer all of your OA (70K) to SA, it will only give you an additional $2800 of interest annually.

So ask yourself this, is $4420 of interest accruing on your loan annually more than $2800 of interest that you earn annually?

Note that the OA to SA transfer is irreversible. You may end up having to fork out some cash to service your mortgage. Do you want to remain cash-rich or cash-poor? Still need a full understanding of your financial situation before giving a recommendation...