Which compressed air and fluid are generally used by oleos? Which compressed air and fluid are generally being used by oleo-struts?
I'm sure it varies by aircraft type, but for the C172S nose gear oleo (for example) it's air and hydraulic fluid. The operating handbook says:


  Keep strut filled with MIL-H-5606 hydraulic fluid per filling
  instructions placard, and with no load on the strut, inflate with air
  to 45.0 PSI