It may be possible to get into a reputable MS program in the US (or overseas) after having done an online Bachelors, but only if the online bachelors was from a reputable, accredited program. Stick to online programs that are attached to reputable, brick-and-mortar universities. Stay away from U Phoenix, Devry, and other schools that have a disclaimer like, “not all credits may transfer”, as that will seriously limit your ability to get into grad school.

Can you not attend a reputable university in Sri Lanka, and obtain your BS there? Many US grad schools – all, in fact – accept foreign students into their MS programs, so long as you meet certain requirements. In fact, I suggest that you check the web pages for your MS program at a university like Fordham University, and see exactly what they require of an international student in terms of undergraduate preparation. You can also call their admissions office and speak to them directly (but that may be cost prohibitive). I’ll be there’s a way for you to get your BS in Sri Lanka, from an appropriate school, and then go on to an MS program in the States.

Online programs are not always recognised, nor are they always reputable. Do be cautious.