O.L.S. is the acronym for a clandestine operation.
It stands for Operation Large Scotch and while that name might give the impression it is a humorous endeavor, be aware that is not the case; there is nothing at all amusing or silly about the plans of rogue elements of the IRA, angry with the Peace Accords and determined to not abide by them, to harm one of Scotland's main industries - Whisky. Once that plot was squashed, however, the acronym persisted to encompass the operations of some of the main participants.
With the explanation out of the way ...
Armitage Brown is an Assistant Controller at MI5.
Richard Hartley is an Assistant Controller at MI6.
Mhairi McClure is an operative associated first with the IRA and later with MI6.
Here is a bit more intelligence on the three and how they are connected:
Brown is the highly experienced - thirty years on the job - spymaster at MI5 who is put in charge of stopping the faction of the IRA who is out to seriously cripple the Scotch Whisky industry as a way of striking back at the United Kingdom. He has devoted his adult life to his craft and has chalked up a good number of successes in that time. The O.L.S. mission will amount to his swan song; though he will be victorious at it, it will bring a cost to him in his health.
Hartley is a former MI5 operative, trained by Brown, who had, some time before we meet him, decided to leave one agency for another, joining MI6 to "broaden his horizons". He will be brought into this collections of related adventures when it is learned by the now retiring Brown that the one key IRA participant in the O.L.S. mission to escape capture was still very much up to shenanigans of some sort, told in the second tale, and since it taking place outside the U.K., Brown asks that Hartley at MI6 be put on the hunt.
McClure is that 'one who got away'. It is important to note that her participation in the O.L.S. affair was not politically motivated (IMHO) but more aimed at one MI5 individual for whom she had a very personal (and justified) hatred. As readers of these adventures will see quickly on, she is a very capable individual able to take care of herself and to cause trouble to her enemies.
The four (so far) adventures collated under the O.L.S. banner are really separate events. In the first, Brown goes to stop an IRA plot, which includes McClure. In the second, Brown asks Hartley to find out what McClure is up to and bring her to justice, if possible. In the third, Hartley is out for revenge at the death of an operative on a different mission and has great use of someone of McClure's skillset. In the fourth, guns are being provided to a myriad of terrorist groups during the Arab Spring and Hartley wants to know by whom and again McClure is too good of an asset to not make use of.
So, O.L.S. might be the name for the first mission but it has proven - by the author, the publishers, and myself - to be a convenient term for several.