If you run Csound from the command line with the option -z, you get a list of all opcodes. Currently (Csound 5.13), the total number of all opcodes is about 1500. There are some overviews and outlines for giving the user some help to find the opcodes which he needs for his task; particularily the Opcodes Overview and the Opcode Quick Reference of the Canonical Csound Manual .
This is another attempt to give some orientation. Compared to the above mentioned ones, not all opcodes are listed, but the listed ones are commented briefly. Some opcodes appear more than once, which is done with intent, because there are different contextes in which you need the ftgen opcode, for instance. As the outline here is different from the overviews mentioned above, you may be able to find better or worse what you are looking for. So use this guide together with other sources and you should be able to find what you need.
Compare Standard Filters and Specialized Filters overviews.
fluidEngine fluidSetInterpMethod fluidLoad fluidProgramSelect fluidNote fluidCCi fluidCCk fluidControl fluidOut fluidAllOut
FLTK overview here
event_i / event scoreline_i / scoreline schedkwhen seqtime /seqtime2 timedseq