Okay, ignore any simple starts-with/contains filters.
I thought you only wanted to match when there was a single identifier in the string, so no multiple values and no comma-separation?
"...need only that user that has got only one account starting with "o" and they should not have any other account numbers."
In which case, the ^ at the start will ensure that the following character occurs at the beginning of the string and the $ at the end will ensure that there's nothing after the preceding matched characters - which in this case are specific enough to only match a single account number.