It is not possible to do local FCY online transfer between local banks in DBS or in OCBC. All FCY transfers in SG are in fact international TT, the problem with DBS and OCBC (maybe other banks too) is that it is not even possible to submit online international TT for destination account in Singapore. It is possible in the branch though (expensive). It is possible to do it from Standard Chartered Bank and if you have World Partner status with them, the only fee (if you choose SHA transfer) is cable fee of US$15. I did this many times between SC and Citibank (EUR to EUR in this case) and always got money in full on Citi side, while the only charge from SC was US$15.
It appears that you can also submit international TT in FCY to other local bank via Citibank Online as you can simply add international recipient with Singapore as destination country.
It appears that you can also submit international TT in FCY to other local bank via Citibank Online as you can simply add international recipient with Singapore as destination country.