Open A (E A E A C# E) strums an A chord on the open strings. Same intervals as Open G, two frets higher; slide players use both.
Below are common chords in Open A, each a full voicing with the root, third and fifth. Frets read low string to high; x is muted, 0 is open, and fr marks the starting fret.
3 3 3 3 3 35 5 5 5 5 57 7 7 7 7 78 8 8 8 8 8x x 3 2 1 30 0 0 0 0 01 1 1 1 1 1x 0 0 3 3 02 2 2 2 1 25 5 5 5 4 57 7 7 7 6 79 9 9 9 8 9Tune with a tuner that knows the target, not one that guesses the nearest standard note. Chartless has Open A built in: pick it, strum, and each string shows how far it is from the Open A pitch.
Chartless listens to whatever is playing and shows the chords live. Set the tuning to Open A and every chord in the chart is shaped for it, so you can play songs you already know in the new tuning the same day. The live chart and tuner are free.
Other tunings: Drop D · DADGAD · Open G · Open D · Double drop D · Drop C · Open E · Open C · Open D minor
More: every tuning, with the notes · capo chart · Nashville numbers · why your guitar sounds out of tune.