#!/bin/bash
s='_OA ............. [6712227000168]
_OA Tasdsd, OA .. [91][355016]
_OA Tasdsd, DA .. [91][5512987000]'
awk '/_OA/ { match($0,/\[[[:digit:]]{4,13}]/);print substr($0,RSTART+1,RLENGTH-2)}' <<< "$s"