I would say they are similar mindsets.

"The basic difference between an ordinary man and a warrior is that a warrior takes everything as a challenge, while the ordinary man takes everything as either a blessing or a curse."

"It is used to describe an attitude in which one sees everything that happens in one's life, including suffering and loss, as good."

I'd say the main characteristic of both schools of thought are not to categorize events, as fortunate or unfortunate, instead seeking to find opportunity in them.  

50th Law explains it a little bit better.. 

"Opportunism is in fact a great art that was studied and practiced by many ancient cultures. The greatest ancient Greek hero of them all, Odysseus, was a supreme opportunist. In every dangerous moment in his life, he exploited some weakness his enemies left open to trick them and turn the tables. The Greeks venerated him as one who had mastered life’s shifting circumstances. In their value system, rigid, ideological people who cannot adapt, who miss all opportunities, are the ones who deserve our scorn—they inhibit progress.

Opportunism comes with a belief system that is eminently positive and powerful—one known to the Stoic philosophers of ancient Rome as amor fati, or love of fate. In this philosophy every event is seen as fated to occur. When you complain and rail against circumstances, you fall out of balance with the natural state of things; you wish things were different. What you must do instead is accept the fact that all events occur for a reason, and that it is within your capacity to see this reason as positive. Marcus Aurelius compared this to a fire that consumes everything in its path—all circumstances become consumed in your mental heat and converted into opportunities. A man or woman who believes this cannot be hurt by anything or anyone."