mplayer is a third party program. You'll find documentation and additional information at the mplayer website:
http://www.mplayerhq.hu
If you have specific questions about the OpenBSD implementation of mplayer, such as how sndio(4) can be used with it, we might be able to help.
As for "hesitation" every few seconds -- it may be the video or audio output choice you are using. I have also seen stuttering on an Intel Atom, single core, when using Hyperthread and the GENERIC.MP kernel. Using GENERIC (bsd.sp) instead cleared that up for me.