Oral presentation or poster presentation, is it always our choice? (paper submission)
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.