Open A tuning
Open A tuning is less common than Open G and it also put more pressure on the strings and the neck. You can emulate the Open A by putting a capo on the second fret when your guitar is tuned to Open G.
Otherwise, the tuning for Open A is this: EAEAC#E. The strings that differ from the standard should be turned so the string tension is decreased and not the opposite.
The Open A is quite popular in blues and was used a lot by John Lee Hooker. A collection of chord with diagrams will here follow.
Including an octave perspective, the tuning is written E2-A2-E3-A3-C#4-E4, meaning that the lowest string is a E note on the second octave, the second lowest string is a A note on the second octave and so on. Standard tuning reference: E2-A2-D3-G3-B3-E4. Open G tuning reference: D2-G2-D3-G3-B3-D4.
Major chords (movable shapes)
The shapes are movable and you have only to lay your fingers across the fretboard. As can be seen in the diagrams, the lowest string is not included but there is an option to do this as well. By playing 000000 instead for X00000 the chord spelling will be A/E.
List of major chords in open A:
G: X 10 10 10 10 10
See below for pdf chord chart ("Essential Chords in Open A Tuning ebook") for many more movable chord, voicings, progressions etc.
Minor chords (movable shapes)
Major chords can be extremely easy in this tuning, but the minor chords are trickier. You probably recognize the Am-shape here, but besides the fingers in the Am-shape you add two notes on the bass strings by placing the thumb over both the strings. You can leave out the 6th string to avoid playing the chords inverted.
You could instead use open position chords, which in most cases are more natural for the fingers:
Minor chords (open positions)
7th chords (movable shapes)
One way to play a blues shuffle is using these movable shapes (examples for D):
This was an introduction about chords in open A tuning. If you want more chords and progressions as well of this particular guitar tuning, see Essential Chords in Open A Tuning with over 300 chord diagrams (ebook with pdf chord charts).
Alternative Open A tunings
Besides the E-A-E-A-C#-E version presented here, the Open A tuning could also be configured as E-A-C#-E-A-E. This version deviates from the common featured also seen in Open D, Open E and Open G with the possibility to play a 5th chord on the three lowest strings. Possible, but not very popular is the Open Amaj7 tuning (E-A-E-G#-C#-E), which has the advantage of only include the A note once.