Leo, Theo, Mateo: the names that end in O

In England and Wales in 2025, 18,054 boys were given a first name ending in O. That is one boy in 15, and three names are nearly half of them: Leo, third in the country with 3,278 boys, Theo 12th with 2,398 and Arlo 15th with 2,083. Then Hugo 46th, Enzo 84th, Milo 90th and Otto 117th. Seven boys in ten with an O name here have one of four letters or fewer.

America has the O too, one boy in 20 in 2017, but a different O. The biggest American O names that year were Mateo, Leo, Santiago, Leonardo, Diego and Antonio, and only one American O boy in five had a short one. In America the O is still climbing: two O names in the boys' top 100 in 2015, eight in 2025. Girls hardly get the O in either country. In 2008 not one girls' name ending in O was in the American top 1,000, and here it is one girl in 225.

How we counted

A name counts if its last letter, as the official file spells it, is O. That is spelling, not sound. Theo counts and Theodore does not. Leo counts and Leon does not. A short O name is one of four letters or fewer: Leo, Theo, Arlo, Hugo, Enzo, Milo, Otto, Nico, Rio. Boys and girls are counted from their own lists. For 1880 to 2008 we use the long American series, which holds the top 1,000 names of each sex and their share of births, so those figures are a floor: a name outside the top 1,000 is not in them. For 1990, 2000, 2005, 2008, 2010 and 2017 we have the full files, every name given to five or more babies, and for 2015 to 2025 the published rankings. England and Wales is the full Office for National Statistics file for 2025, checked against 2024. There is no hand list.

Otto, then Leo, then Antonio, then Diego

In 1880 there were 31 names ending in O in the American boys' top 1,000, and one boy in 128 had one. Otto was 63rd in the country and nearly a third of the group on its own. Leo was 93rd, Alonzo 110th, Lorenzo 240th, Pedro 304th, Antonio 334th, Milo 339th, Hugo 352nd, Francisco 395th, Santiago 558th, Theo 680th, Romeo 734th and Bruno 756th. Most of Britain's O names of 2025 were on that list, a long way down. Arlo was not; it first appeared in 1900, at 907th.

Leo passed Otto in 1888 and was the biggest O name in America for 72 years. Its best year was 1903, 37th in the country, and it was still 48th in 1920. Then it fell for the best part of a century: 63rd in 1930, 112th in 1940, 161st in 1950, 231st in 1960, 370th in 1980 and 485th in 1995, its worst year. Otto went the same way and further, 206th in 1920, 552nd in 1950 and out of the top 1,000 after 1974. Theo was last listed in 1945, Arlo in 1944, Milo missed the 1990 and 2000 lists altogether. The low for the whole group was 1942, when one American boy in 149 had an O name, and the only O name above 250th was Leo at 120th.

What brought the O back in America was not those names. From 1960 the leading O name was Antonio, which went from 196th that year to 71st in 1976, and behind it came Mario, Ricardo, Roberto, Francisco, Fernando, Eduardo, Alejandro and Sergio. One boy in 102 in 1960, one in 66 in 1970, one in 47 in 1980, one in 39 in 1990. Diego took over as the leader in 2003 and was 56th in 2006, the group's record year, when one American boy in 28 had a name ending in O and the eight biggest were Diego, Antonio, Alejandro, Eduardo, Fernando, Ricardo, Francisco and Mario. In the full 2008 file, 91,485 boys had an O name, and 7,072 of them, one in 13, had a short one.

2017: the two lists meet

By the 2017 file, the last full count we hold, 92,582 American boys had a name ending in O, one in 20, across 768 spellings. The biggest was Mateo with 7,726, which had first entered the top 1,000 in 1995. Then Leo 5,923, Santiago 4,188, Leonardo 4,113, Diego 3,088, Antonio 2,470, Matteo 2,071, Alejandro 2,027, Lorenzo 1,994, Emiliano 1,864, Thiago 1,765, Milo 1,688, Enzo 1,419, Francisco 1,398, Emilio 1,383, Theo 1,223, Eduardo 1,221, Ricardo 1,160, Fernando 1,134, Mario 1,129 and Arlo 1,084. Short O names were 19% of the group, up from 5% in 1990 and 8% in 2008, and Leo was a third of the short ones on its own.

The rankings since then say the same thing from both ends. Mateo went 85th in 2015, 42nd in 2017, 20th in 2020, 6th in 2023 and 8th now. Leo went 91st, 62nd, 36th, 18th and 19th. Santiago 127th to 26th, Thiago 334th to 50th, Enzo 330th to 72nd, Theo 406th to 82nd, Emiliano 230th to 94th, Lorenzo 216th to 117th, Milo 288th to 119th, Arlo 500th to 148th, Nico 464th to 174th, Otto 544th to 277th. In 2015 the boys' top 100 held two O names. In 2020 it held four and in 2025 eight: Mateo, Leo, Santiago, Thiago, Enzo, Theo, Leonardo and Emiliano. The top 1,000 holds 89, up from 78.

The O names of 1990 are the ones falling. Antonio was 77th in 1990 and is 176th. Alejandro 89th in 2000, 195th now. Diego 125th in 2015 to 158th, Francisco 247th to 322nd, Eduardo 251st to 410th, Ricardo 278th to 380th, Fernando 272nd to 387th, Mario 284th to 438th, Sergio 332nd to 441st, Roberto 358th to 570th. Alfonso, Rodolfo, Rogelio and Rolando have left the top 1,000 since 2015. So the American O is not one wave but two: the names of the 1970s and 1980s going down, and Mateo, Santiago, Thiago and Emiliano going up alongside Leo, Theo, Enzo, Milo and Arlo.

England and Wales: one boy in 15

In England and Wales in 2025 the O is commoner than it has ever been in America: 18,054 boys, one in 15, across 301 spellings. Leo, Theo and Arlo are 43% of them. Then Hugo 1,094, Enzo 617, Milo 597, Otto 444, Leonardo 389, Lorenzo 378, Mateo and Matteo 328 each, Nico 306, Rio 305, Romeo 260, Lando 217, Rocco 192, Kairo 187, Santiago 153, Elio 147, Kylo 138, Mylo 136 and Thiago 128. Six O names are in the top 100 here against America's eight, and three are in both: Leo, Theo and Enzo. Of the 12,603 British O boys with a short name, 7,759 are Leo, Theo or Arlo. 2024 was the same: one boy in 15, Leo 5th, Theo 12th, Arlo 15th, and seven in ten short.

The American O barely crosses. Mateo, 8th in America, is 160th here with 328 boys. Santiago, 26th there, is 296th here. Diego went to 87 boys, Antonio 69, Mario 37, Francisco 29, Alejandro 17, Emiliano 20, Ricardo 20, Eduardo 12, Fernando 10 and Maximiliano 8. Going the other way, Theo is 12 times as common per head here as in America, Arlo 11.7 times, Hugo 9 times, Otto 4.4 times and Leo 3.4 times. Rio is 14.7 times, with 305 boys here and 127 there. Antonio is 5.9 times as common there, Diego 5.8, Santiago 4.5, Mateo 3.9. Taking every O name together, the two countries are close: boys are 1.2 times as likely to get one here.

The girls: Cleo, then Jo, then nobody

Girls have never had the O. In 1880 one American girl in 3,200 had a name ending in O, and the biggest was Cleo at 406th. Cleo led until 1925 and then Jo took over: 131st in 1930, 73rd in 1935, 51st in 1947, when it was four fifths of the girls' O on its own, and 58th as late as 1955. The group's best year was 1954, one girl in 212. From there it fell to one in 1,800 by 1980, by 1986 Rocio was the only O name left in the top 1,000, and in 2008 there was none at all. The 2017 file has 2,528 girls, one in 677, and the biggest names are Margo 235, Indigo 167, Cleo 151, Halo 149, Cielo 143 and Lucero 137. On the rankings the girls' top 1,000 went from no O names in 2015 to five in 2025, all below 550th.

England and Wales 2025: 1,125 girls, one in 225, and Cleo is 423 of them, 114th in the country and 17 times as common per head as in America. Then Margo 120, Juno 104, Indigo 82 and Coco 50. Girls are 2.7 times as likely to get an O name here as there, which is a bigger gap than the boys have, and it is nearly all Cleo.

What we cannot tell you

Why. The files hold what parents chose, not why, and a letter does not have a reason. We can say that the O names of 1880 fell for ninety years and came back, that America filled the gap with a different set of O names from the 1960s and is now giving those up, and that Britain went straight to Leo, Theo and Arlo. We cannot say what any of that was copying. The rule counts a letter, so it puts Theo with Thiago and Otto with Antonio, and it leaves out Theodore, which is 4th in America and 9th here and is where many of the Theos are. We do not hold American counts after 2017 or any year from 2009 to 2014, the long series is a floor because it holds only the top 1,000, and we hold two British years, so Britain's own history of the O is not in this article.

What it means for you

If you like the O, you are choosing from two lists, and which one you are on depends on where you live. Leo is the safe one: third here, 19th there, climbing in both, and the biggest O name in America for 72 years once already. Theo, Arlo and Hugo are Britain's, nine to twelve times as common here, and Otto four times; Theo, Arlo and Otto are climbing in America from a long way back, and Hugo is not moving. Mateo, Santiago and Thiago are America's, top 50 there and outside the top 150 here. Enzo, Milo and Nico sit in between, top 175 in both. For a girl the O is yours for the taking: one girl in 225 has one, and after Cleo nothing has more than 120.