As Oliver Cowdery took down Joseph Smith’s dictation in 1 Nephi 2:9, he accidentally skipped “the fountain of ” when he initially wrote “continually running into all righteousness”. Oliver immediately corrected the text by supralinearly inserting “the fountain of ” (without any change in ink flow). It would be very difficult to imagine this correction as some kind of emendation on Oliver’s part.
The phraseology “the fountain of all righteousness” is used elsewhere to refer specifically to the Messiah (which gives a Christological interpretation to Lehi’s wish for Laman):
Summary: The supralinear insertion in 𝓞 of “the fountain of ” in 1 Nephi 2:9 is not an emendation but instead represents the reading of the original text.