Is digital camera makers pure competition or oligopoly?

No. Depending on how you count them, there are at least a half-dozen to a dozen manufacturers of digital cameras (Canon, Nikon, Olympus, Sony, Samsung, Panasonic, Pentax, maybe HP, Casio, Leica, Ricoh, Fuji). Include mobile phone makers like Apple that have taken a big chunk of from the point-and-shoot makers (RIP Kodak, Minolta, Yashica, Konika) and there are too many players for an oligopoly. The number hasn't changed that much in the past 20-30 years.