Oh Shenandoah is a gorgeous melody, very evocative, with a feeling of longing and homesickness (in a good way) that you can hear in the melody even if you don't know the lyrics.
If you keep this feeling of longing and wide-open-spaces as you are playing these Oh Shenandoah tabs, you'll get it right.
One thing to look for: the word "Way" is broken up melodically, the first time into three syllables, the 2nd time into four - that's why you see the lyrics in the Oh Shenandoah tabs broken up phonetically as they are below.
Here's the Oh Shenandoah tabs played on a C diatonic harmonica:
OH SHENANDOAH
3 4 4 4 -4 5 -5 -6 Oh, Shenandoah, I long to hear you,
6 7-7 -6 6 -6 6 5 6 wa-aa-ay, you rolling river,
6 -6 -6 -6 5 6 5 -4 Oh, Shenandoah, I long to hear you,
4 3 4 -4 5 wa-aa-aa-aay,
4 5 -6 6 we're bound away,
4 5 5 5 4 -4 4 ah Cross the wide Missouri.
Here it is again played guitar/harmonica (harmonica in a rack) by E Matthew Shelton Please note: This is played on a key of G harmonica, guitar in G, and there is some improvising of the melody