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Break these two down. Pronounce "o-" and "-nee-" in the same way described above. Pronounce the "sa". Put it together.
"Oneesan" is more polite, due to the "o-". "-san" is respectful. You would use it with people socially equal to you or with someone you don't know very well. The "-sa-" in "-san" is pronounced the same as in "-sama". This is one great thing about Japanese: sounds are consistent and don't vary between words, with a few exceptions. The Japanese "n" sounds like the English "n". If anything, it sounds a little like the English "m", depending on the speaker.