Simao, sorry I should have been more definitive about the reboot process. Any or all of the buttons will work. The idea is to drain any stored capacitance which will be discharged by pushing buttons. That lack of any stored capacitance or energy which can cause micro processors to keep even Minute memory stored and cause disfunction, will be erased. So upon plugging the unit back in the processors will restart in their default programming.
However if OPPO didn't recommend this, it may not work. But no harm can be done by trying. This procedure works most anything with processors. I have even seen it work in older cars by disconnecting the battery. A lot of people recommend a similar process but say leave it unplugged for 5-10 minutes and it can discharge the processors as well. Just by pushing buttons, it forces the processors to drain as it will try to utilize any stored energy to fulfill the requested command.
I have a Maytag washer that I have to do this on occasion and actually have the beeper emit a tone and light the panel 2-3 times while unplugged.
Good luck and let me know what results you have.