Volume control in the BDP-105 is implemented in the DAC chip as a 32-bit floating point multiplier. When the volume is set to 100, the digital audio signal is not attenuated at all. The headphone amplifier gets the maximum input level and drives the headphones at the rated output (depending on the impedance of the headphones). If volume level 100 works for your headphones, that's the best. If it is not loud enough (unlikely though), then the headphone amp in the BDP-105 is not suitable for your headphones - you will need to use an external headphone amp.
It is most likely that volume level 100 is too loud. You will need to reduce the volume. Will that hurt the sound quality? The answer is no. Thanks to the 32-bit volume control in the DAC, the performance is as good as an analog volume control but without the statics and wear.
ESS has a nice presentation about the volume control mechanism in their DAC chips, which are used in the BDP-105.
The second to last slide says, "A digital volume control with access to the DAC internal data path will behave just like the analog one until it reaches the noise floor of the analog components of the DAC." The noise floor is at -135dB in the ESS Sabre DAC. It is nothing if you reduce the volume 20dB or so, which is volume level 60 on the OPPO BDP-105.