[6] Interruptions to their ascendancy have occurred rarely, mainly in the two decades after the Second World War from 1946 to 1965 when five other clubs won the league, and in the first half of the 1980s with the challenge of the New Firm of Aberdeen and Dundee United.

[11] The origin of the term is unclear but may derive from the two clubs' initial match in which the commentators referred to the teams as "like two old, firm friends",[12] or alternatively may stem from a satirical cartoon published in 'The Scottish Referee' sports newspaper prior to the 1904 Scottish Cup final between the sides, depicting an elderly man with a sandwich board reading "Patronise The Old Firm: Rangers, Celtic Ltd",[13] highlighting the mutual commercial benefits of their meetings.

[14][15] The name may also be a reference to these two teams being among the original eleven members of the Scottish Football League formed in 1890 [16] (although others from that group, such as Heart of Midlothian and St Mirren, also continue to play at the highest level to this day).

Rangers' traditional support was largely from the Protestant community, and for decades the club had an unwritten rule whereby they would not knowingly sign a player of the Catholic faith.

[21] The policy was decried by Graeme Souness when he became manager, and he brought ex-Celtic forward Mo Johnston to the club in a very public move away from the practice, which no longer continues.

[22][23][24][25] Celtic's support was largely from those of Irish Roman Catholic backgrounds and while the club practiced no exclusion of Protestants and signed many of them to play for the team, there was a pro-Catholic mindset among some of the employees.

[30][29] In that era Rangers had won three successive championships and expanded their stadium at great expense, only for one of the new wooden stands to collapse during a Scotland v England fixture in April 1902, killing 25 and injuring hundreds of others.

[31][32] The disaster forced the club to rebuild Ibrox for a second time and financed this by selling off their best players, with Celtic, in particular, taking advantage of the weakness to win six successive titles between 1905 and 1910 before Rangers returned to their previous strength.

Hundreds of Ulster Protestant workers, many of Scottish descent, also made the move, and they adopted Rangers – the closest large club to the Govan yard – as their new team.

[25][30][36] Other events such as World War I and the Easter Rising contributed to the club being adopted as a symbol of the Scottish establishment and of British Unionism in the face of Irish Catholic rebellion personified by the success of Celtic[37][25][30] and from that time on, many across Scotland and Northern Ireland (and the diaspora of those communities in England, North America and elsewhere) became supporters of Rangers or Celtic over and above their local teams according to their own political and religious leanings, including polarised attitudes towards 'The Troubles'.

[44] In 2015, former Rangers player Brian Laudrup said that the Old Firm topped all of the rivalries he had played in,[45] which included the Milan derby and the Fiorentina-Juventus meetings[46] in Italy; ex-Celtic striker Henrik Larsson, who experienced El Clásico in Spain and De Klassieker in the Netherlands, has made similar comments.

[3] Jim Bett, who had already played in Iceland prior to joining Rangers in the 1980s and thereafter moved to Belgium, stated that he declined an opportunity to return to the Ibrox club due to the sectarianism associated with life as a footballer in the west of Scotland, in contrast to his positive experiences living abroad.

[47] Hundreds of opposing fans fought an on-pitch battle in the aftermath of Celtic's 1–0 victory in the 1980 Scottish Cup final at Hampden, fuelled by alcohol and armed with the cans and bottles.

In 1987, four players were charged by the police with breach of the peace for their conduct during a match at Ibrox and had to appear at court,[54][61][62] with two (Chris Woods and Terry Butcher) convicted and fined.

[54][64][65] Gascoigne, who had pleaded his ignorance of the situation after he made the same gesture in a friendly just after joining Rangers in 1995[66] and had been sent off on his last visit to Celtic six weeks earlier,[67] was fined for the provocative act[68] and left the club later that year.

[71][72] In February 2006, Celtic goalkeeper Artur Boruc was cautioned by the police for gestures he made to Rangers supporters during a match at Ibrox; six months later, it was clarified that this was for "Conduct which appears to incite disorder" rather than simply making the sign of the cross as he entered his area, as some had thought.

[77][78] Prior to an international match between Northern Ireland and Poland in 2009, graffiti of a threatening nature mentioning Boruc appeared on walls in a Rangers-supporting area of Belfast.

[54][80][81] A 'crisis meeting' was convened involving the clubs, the Scottish Government and Strathclyde Police some days later regarding the trend of violence among supporters away from the pitch increasing on Old Firm matchdays and concerns that incidents during the matches was a factor.

[82] The incident occurred during the tense environment of a season where seven Old Firm matches took place (including a League Cup final and a fight for the title eventually won by Rangers by one point).

[87][88][89] Towards the end of the 2022–23 season, a dispute between the clubs over ticketing and supporter safety issues led to there being no away fans at all for the last two league fixtures, which was also the case throughout 2023–24[90] and, despite an agreement being reached in March 2024 to end the stalemate and allow a 5% (~2,500) visiting allocation at each stadium,[91] continued into 2024–25 (the prolongation was blamed by Celtic on the unpredictability of renovations at Ibrox which caused some matches to be played at Hampden and others with reduced capacity, although it was highly likely the work would have been finished by the relevant date of 2 January 2025).

Kelly had described the 2012 legislation as having "completely failed to tackle sectarianism" and as "illiberal" which "unfairly targets football fans", and was "condemned by legal experts, human rights organisations and equality groups".

[7] Some Celtic supporters were particularly vociferous in their assertions, to the extent of a group paying for a full-page newspaper advertisement in January 2015 announcing that their club would soon play its first fixture against the new Rangers.

[128][129][130] It took Rangers four years to climb through the lower divisions and re-take their place in the Scottish Premiership for the 2016–17 season; in the interim only two cup semi-finals were played between the clubs[131][132][133][134] and Celtic won all four league titles by significant margins (never less than 15 points).

[136][137] The 2017–18 season was much the same: Celtic won three of the Old Firm league fixtures[138] plus a Scottish Cup semi-final meeting[139] and lifted all three domestic trophies;[140] Rangers finished third, behind Aberdeen.

[144] Initial signs that the 2020–21 Scottish Premiership (played almost entirely in empty stadiums due to the pandemic) would be closely fought proved inaccurate as Rangers produced consistent and defensively-strong performances, in contrast to Celtic who dropped goals and points regularly as the season progressed (although they did set a new record with a 'quadruple treble', winning twelve domestic trophies in succession).

[231] The 1971 Ibrox disaster – in which 66 Rangers supporters died in a crush on an exterior stairway – occurred at the end of an Old Firm match,[225] although the identity of the opposition was not a factor in the incident other than having drawn a large crowd of at least 75,000 to the event.

[241] The intensity of the rivalry is fuelled by the clubs' historical duopoly in Scottish football, with most meetings between them being pivotal in deciding the destiny of a championship or cup and anything but a title-winning season seen as a major disappointment, particularly as it would usually mean 'the enemy' has won the trophy.

In the early 2020s, despite their economic troubles over the previous decade, Rangers were able to re-establish themselves at close to the same level as Celtic and far ahead of the other clubs, who can only realistically aim to finish in the higher league positions to qualify for European competitions or win a knockout cup – which would usually involve beating at least one of the Glasgow giants.

Rangers' sudden removal from the upper echelons of the Scottish game in 2012 led to a more diverse list of finalists in both cups; the expected Celtic monopoly of all competitions did not occur in the knockout formats and there were several maiden trophy wins for clubs such as St Johnstone, Inverness CT and Ross County.

[303][304][305] The following season began with Rangers claiming their first SWPL Cup[306] and ended with crowds of over 10,000 at both Celtic Park and Ibrox watching the final league fixtures – in which Glasgow City fended off their increasingly well-funded rivals and regained the championship in dramatic circumstances[307][308] – quickly followed by another healthy attendance at Hampden in the first Old Firm cup final in the women's game (as well as the first to be held at the national stadium), with Celtic retaining the trophy.