I have looked this up before and found this:
But have you considered why “Nyanpasu” is a creative coinage of a new term in Japanese?
Japanese people and linguists may disagree on the exact etymology of the greeting “Osu!” This greeting is often used by people who are exerting themselves physically, such as karate students. (“Osu” is customarily used by men, and it is customarily used to greet people who are not older than the speaker. Thus it is a little funky when we see Renge, who is female, using it to address people who are older than she is.)
If we wanted to combine the cat-like meowing sound of “NYAN” to “OSU,” why wouldn’t we just say, “NYAN-OSU”?
Think about the way we say “senpai.” The “p” sound is a voiceless bilabial stop. The “n” sound is a nasal. When the nasal precedes the stop, it is very easy to slur “senpai” into “sempai.”
When using the nasal sound “n” before a word boundary, it is very easy to slur the sounds, and in the case of “nyanpasu” the voiceless bilabial stop replaces the conventional word boundary.
That could be considered funky.
Saying it's a combination of nyan and osu, but one that rolls off the tongue a lot more smoothly.
It seems like a decent explanation to start with.