Without actual logs, it is very difficult to isolate the pattern for your errorcode to write the correct regex. Having said that, if you error codes are always the same patter "OPT-" followed by 6 characters, then you can use this. This also assumes the error code appears only once an event. This also assumes, this pattern of OPT- followed by 6 chars doesn't occur anywhere else in the event. If it does, this regex will only get the first occurrence
| rex "(?<ec>OPT-\w{6})" | timechart count by ec
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