I tried before there is no option for CPF investment online, someone advised and I have checked using CPF OA you will have to go down to the Bank for the application, in the case of DBS have to go to Main Branch, not sure for OCBC. I don't think there is a charge but I am yet to try.
"For CPFIS Application: You will need to submit an application in person at the main branch of your respective CPF Investment Scheme (CPFIS) agent bank (DBS/POSB, OCBC, or UOB)."
I am thinking for the coming 5/10 years bond but for T-Bills if there is only marginal difference interest rate and I personally don't think its worth with the effort as the tenture is short.