Build Up procs have a duration of 5.25s.
OSA has an animation time of 1.32s.
Spawning OSTarget requires an additional 0.132s.
Presuming you're using a relatively fast Fire or Energy attack, it'll take you 1.0-1.32s to ignite OSA.
Spawning OSBurn tacks on another 0.132s.
So discounting human response time, out of that 5.25s of Build Up boost duration, OSA will deal increased damage for 2.34-2.67s. Including human response time, you're probably looking at 2s at the most.
I wouldn't go this route, but it's your character, your call.