I believe it's opengl, I'm not sure what ways exist to find out what the current vo is, when you start mpv from cmd shell (type either mpv or mpv.com) it prints the vo along with some other info to the shell. What I found the shell most useful for so far is debugging scripts as it also prints errors in scripts and config files.
Code:
D:\Software\Medien\Player\mpv>mpv D:\Temp\test.mkv
Playing: D:\Temp\test.mkv
(+) Video --vid=1 (*) (h264 1280x720 29.970fps)
(+) Audio --aid=1 --alang=ger (aac 2ch 44100Hz)
AO: [wasapi] 48000Hz stereo 2ch float
VO: [opengl] 1280x720 yuv420p
AV: 00:00:17 / 00:01:41 (17%) A-V: 0.000