Technically, if you had the right kind of adapter, you could use the rear L/R RCA outputs into your headphones.
The type of adapter would depend on the input(s) of what headphones you are using, but it shouldn't be an expensive solution.
Oppo does also make the BDP-105 and BDP-105D that have a very nice built in headphone amps.
They also sell a stand alone dedicated 2-channel headphone-amp/dac/pre-amp called the HA-1.
Also, the PM-1 and PM-2 headphones. https://www.oppodigital.com/
For more reference, here are the thread dedicated to the BDP-103,
and if you ask there, you will most likely get the same type of response,
but maybe with some variation. More opinions are always better to help make an informed decision. Official OPPO BDP-103 Owner's Thread