I submitted a paper for a conference and I was just wondering since it's a paper submission is it sure that it will be an oral presentation or they can still ask for a poster presentation?
That would depend on the conference itself. You need to read the fine print or ask the program committee. Some will either just accept or reject a paper (as corrected, of course). Some have separate poster sessions with separate committees and a different criteria for acceptance. There is too much variability to make a clear answer.
However, you should not depend on that happening at any conference. It is possible that if your paper is rejected, one of the reviewers might suggest submitting it as a poster. That of course would depend on due dates.
The official conference scientific committee, frequently supported by a selected team of peer reviewers -- ...and a spoonful of politics... -- will ultimately decide on whether a submission should feature as oral presentation or a poster. Or not to feature at all (yes: conference submissions can be rejected).
Generally, oral presentations are regarded as "of higher status" than posters, thus fitting in as oral presentation might be more competitive.