Note that usually you are also using the microscope to observe what you are trapping, so it' super convenient (but not easy), to use the same objective that the one you observe to send a laser through (in the opposite direction) and trap your sample. If you send your laser-beam quasi parallel it will focus and trap particula quasi-where your are observing.

(source: did my PhD with optical traps: http://matthiasphd.herokuapp.com/html/parts/part1.html#optic..., picture of trapped beed as viewd through microscope: http://matthiasphd.herokuapp.com/html/parts/part3.html#exper...)