It doesn't matter how you booked your flights, and nor do you have to be a Krisflyer member.

I've never had a problem doing OLCI on SQ flights booked as LH awards nor SQ flights bought through interlined ticket stock. OLCI is available to anyone 48 hrs prior to departure unless you're departing from CMB. It is very helpful to have ur SQ PNR handy so make sure you get that from UA or SQ.