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$ oa index --help
usage: oa index [-h] [--single] [--mate-pairs] [--check-ids]
                [--max-read INDEX:MAXREAD] [--length INDEX:LENGTH]
                [--estimate-length FRACTION] [--fasta] [--no-pipe]
                [--forward-fasta] [--check-pairing] [--reverse-fasta]
                index forward [reverse]

positional arguments:
  index                 name of the produced index
  forward               Filename of the forward reads
  reverse               Filename of the reverse reads

optional arguments:
  -h, --help            show this help message and exit
  --single              Single read mode
  --mate-pairs          Mate pair library mode
  --check-ids           Checks that forward and reverse ids are identical
  --max-read INDEX:MAXREAD
                        the count of million of reads to index (default the
                        full file)
  --length INDEX:LENGTH
                        The length of the read to index (default indexed
                        length is estimated from the first read)
  --estimate-length FRACTION
                        Estimate the length to index for conserving FRACTION
                        of the overall dataset
  --fasta               sequence files are fasta formated
  --no-pipe             do not use pipe but temp files instead
  --forward-fasta       forward file is a fasta file
  --check-pairing       ensure that forward and reverse files are correctly
                        paired
  --reverse-fasta       forward file is a fasta file

The last line should read:

  --reverse-fasta       reverse file is a fasta file

UPDATE: Actually corrected forward to reverse.

(I'm not sure why you need --fasta plus --forward-fasta and --reverse-fasta)