Should I invest using cpf? If yes when should I start? Before or after buying a house?

You cannot invest the first 20k in OA. Your investment might be at a loss depending on what and when you invest initially, so it depends on your priority.

Not keen to invest cpf money on nonguaranteed instruments. only tbills that can give higher than oa yield

No.

Not a fan of investing CPF money even though if you have decent investing knowledge and track record.

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It is all about portfolio management, CPF is a risk-free asset that gives you ranging from 2.5% to 5% returns, which is something that you can't find in the open market.

In my case at the age of 31 years old, my CPF stands around 30+% of my personal portfolio, which is a very comfortable proportion in my asset portfolio, which also means that I do not need to invest in any low risks investment to build up my retirement sum.