Well, what i'd do is use sin/cos. Have an angle variable, and constantly add values to it.
Graphics 800,600 setbuffer backbuffer() Global angle Global x#,y# x = 300 y = 200 While Not KeyHit(1) cls angle = angle + 1 oval x,y + (sin(angle)*200) ; 200 is just a number to multiple the effect enough to see it. flip wend end