How do I disable the mode for using j-k keys to scroll through history?

[Steve Dondley <s@xxxxxxxxxxx>]

I've been using zsh with oh-my-zsh for several months now with vi 
commands. Frequently, the command line seems to "lock up" and become 
unresponsive to input. I chalked it up to some kind of bug, hit ctrl-c, 
and moved on.

I finally just figured out that I had somehow entered some special mode 
that activates the "j" and "k" keys to pull up prev/next commands. 
Through trial and error, I discovered the <esc> key triggers this mode 
and the "s" key gets out of it.

Haven't figured out how to disable this feature. Google has turned up 
nothing useful after 10 min. of searching. Any ideas?