otx stands for "object tool extended". It uses otool (object tool) to disassemble a Mach-O executable file, then enhances the disassembled output. Simple enhancements include adding the machine code of each instruction and the offset of each instruction from the beginning of a function. More complicated enhancements include displaying the names and data types of Objective-C methods even if symbols have been stripped, and adding comments that describe member variables, function calls, static data and more.
otx users should have Xcode 5 or above installed. The otx distribution includes both a GUI application and a command line utility, for your convenience. You can use either or both, as you see fit.