FTP logs are largely in xferlog format. That follows this convention:
current-time transfer-time remote-host file-size filename transfer-type special-action-flag direction access-mode username service-name authentication-method authenticated-user-id completion-status
In your case, I believe you are interpreting 10079, a_or and O*c as three separate pieces of information. Those pieces of information are actually seven distinct pieces of information
10079 is the file size
a is transfer type (a stands for an ascii transfer)
_ is the special-action-flag (_ means no action taken)
o is the direction (o is for outgoing)
r is for access mode (r is for "real" or locally authenticated user)
0 is for authentication-method (0 = none)
* is for authenticated-user-id (* means "not available")
c is for completion-status (c means "complete" for a complete transfer)
Check out man xferlog for more information. Here's a web based man page for xferlog.