[R] extracting from a data.frame

[dax42]

Hi,

I am sorry for this simple question, but... How do I extract something 
from a data.frame?
The following is my Problem:
I have got a dataframe "a" with various columns. One of those columns 
is called V3 and contains elements of the following levels:

 > levels(a$V3)
  [1] "C"   "CA"  "CB"  "CD"  "CD1" "CD2" "CE"  "CE1" "CE2" "CE3" "CG"
[12] "CG1" "CG2" "CH2" "CZ"  "CZ2" "CZ3" "N"   "ND1" "ND2" "NE"  "NE1"
[23] "NE2" "NH1" "NH2" "NZ"  "O"   "OD1" "OD2" "OE1" "OE2" "OG"  "OG1"
[34] "OH"  "OXT" "SD"  "SG"

Now I would like to get all rows of a, that have the value "O" in V3.
I tried a lot of different combinations of [... but I couldn't make it.

Any ideas?
Thanks a lot!
Dax.