Last official estimated population of O'Brien County was 14,400 (year 2014)[1]. This was 0.005% of total US population and 0.5% of total Iowa state population. Area of O'Brien County is 573.2 mi² (=1484.6 km²), in this year population density was 25.12 p/mi². If population growth rate would be same as in period 2010-2014 (+0%/yr), O'Brien County population in 2024 would be 14,405 [0].
Historical population of O'Brien County for period 1860-2014 [1] , [2] , [3]:
Estimated population for 2014[1]. Population density is in p./sq.mi [4]. Changes are for period 2010-2015. Value is %/year. Boundary lines are approx. 2012 [5]. Click on flag to hide it.
Population • Density • Changes • Legends • Position
[1860-1870] +56.72 %/yr
[1870-1880] +19.24 %/yr
[1880-1890] +12.13 %/yr
[1890-1900] +2.66 %/yr
[1900-1910] +0.16 %/yr
[1910-1920] +0.99 %/yr
[1920-1930] -0.34 %/yr
[1930-1940] +0.47 %/yr
[1940-1950] -0.17 %/yr
[1950-1960] -0.07 %/yr
[1960-1970] -0.72 %/yr
[1970-1980] -0.32 %/yr
[1980-1990] -0.94 %/yr
[1990-2000] -0.23 %/yr
[2000-2005] -0.73 %/yr
[2005-2010] -0.21 %/yr
[2010-2014] +0 %/yr
Looking for settlements in O'Brien County? Visit this page: Settlements in O'Brien County, IA
Population of O'Brien County by age groups male (M) and female (F)[1b]
| Age Gr. | (M) | (F) | Total |
| 0 to 4 | 407 | 435 | 842 |
| 5 to 9 | 522 | 459 | 981 |
| 10 to 14 | 518 | 428 | 946 |
| 15 to 19 | 445 | 395 | 840 |
| 20 to 24 | 411 | 343 | 754 |
| 25 to 29 | 353 | 320 | 673 |
| 30 to 34 | 439 | 359 | 798 |
| 35 to 39 | 404 | 363 | 767 |
| 40 to 44 | 345 | 331 | 676 |
| 45 to 49 | 413 | 359 | 772 |
| 50 to 54 | 516 | 533 | 1,049 |
| 55 to 59 | 580 | 531 | 1,111 |
| 60 to 64 | 474 | 477 | 951 |
| 65 to 69 | 340 | 345 | 685 |
| 70 to 74 | 271 | 297 | 568 |
| 75 to 79 | 209 | 294 | 503 |
| 80 to 84 | 205 | 312 | 517 |
| 85+ | 208 | 415 | 623 |
O'Brien County resident population estimates by Sex, Race, and Hispanic Origin[1b]. For full list of abbrevations see: population.us/about page.
| Race | Male | Female | Total |
| WAC | 6917 | 6865 | 13,782 |
| WA | 6854 | 6801 | 13,655 |
| NH | 6694 | 6728 | 13,422 |
| NHWAC | 6586 | 6626 | 13,212 |
| NHWA | 6533 | 6581 | 13,114 |
| H | 366 | 268 | 634 |
| HWAC | 331 | 239 | 570 |
| HWA | 321 | 220 | 541 |
| BAC | 101 | 78 | 179 |
| AAC | 59 | 76 | 135 |
| TOM | 65 | 67 | 132 |
| BA | 78 | 53 | 131 |
| NHAAC | 54 | 71 | 125 |
| NHBAC | 71 | 52 | 123 |
| NHTOM | 55 | 48 | 103 |
| AA | 43 | 58 | 101 |
| NHAA | 41 | 57 | 98 |
| NHBA | 51 | 33 | 84 |
| IAC | 39 | 43 | 82 |
| HBAC | 30 | 26 | 56 |
| NHIAC | 30 | 26 | 56 |
| HBA | 27 | 20 | 47 |
| IA | 16 | 17 | 33 |
| HTOM | 10 | 19 | 29 |
| HIAC | 9 | 17 | 26 |
| NHIA | 11 | 9 | 20 |
| HIA | 5 | 8 | 13 |
| NAC | 9 | 3 | 12 |
| NHNAC | 8 | 3 | 11 |
| HAAC | 5 | 5 | 10 |
| NA | 4 | 0 | 4 |
| NHNA | 3 | 0 | 3 |
| HAA | 2 | 1 | 3 |
| HNAC | 1 | 0 | 1 |
| HNA | 1 | 0 | 1 |
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[0] Calculated by population.us from last known population growth rate, not official data.
[1] County population - 2014, archived from the original, retrieved 02/23/2016
[1b] Annual County Resident Population Estimates by Age, Sex, Race, and Hispanic Origin, (used only data for 2014) archived from the original, retrieved 02/23/2016
[2] County population - 2000-2010, archived from the original, retrieved 02/23/2016
[3] Population of States and Counties of the United States: 1790 to 1992, PDF, 15 Mb, census.gov, retrieved 02/23/2016
[4] County area, latitude longitude data, retrieved 02/23/2016
[5] County boundaries, CSV files on boundaries.us, retrived 03/18/2016
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