USAO has a USA that includes all of the Americas, but the PoD is in 1774. It sees Quebec and the Iroquois join the American Revolution. This causes the American culture in its formative stage to develop a much more inclusive and tolerant view of Catholics, Romance-speakers, and 'civilized' Native Americans, and Canada to willingly join the USA.
Political butterflies (long Federalist hegemony) make the USA much, much better prepared for the War of 1812, where in addition to kicking Redcoat butt in Rupert's Land and the Caribbean (also thanks to events in Europe), the Americans intervene in the Spanish-American wars of independence to support the Patriots. This makes Bolivar and co. become pro-US revolutionaries and Manifest Destiny take a Pan-American character, so Gran Colombia, Cuba, and Peru willingly join the USA. Mexico is later annexed by means similar to OTL.
Having been expelled from North America, Britain conquers the Southern Cone during the Napoleonic Wars and develops a lasting rivalry with the USA. It intervenes during the ACW to support the CSA with France, Spain, Portugal, Brazil, and the Ottomans, but Greater Germany, Italy, Hungary (all formed in the early 1830s), and Russia intervene to support the Union. In the resulting world war, the Union and its *CP allies win, causing the USA to remain united and annex most of British South America and all the Caribbean, leaving only Brazil and UK Araucania-Patagonia outside the USA in the Americas.
The expanded ACW makes the Union embrace Radical Reconstruction, with various reforms that destroy Dixie racial segregation and the peonage system in Hispanic states, ending racism and cementing democratic national unity.
A generation later, Brazil (that has gone neo-Confederate) experiences an anti-slavery insurrection. The USA intervenes to help the abolitionists and annexes Brazil. The Americas are willingly united under US democracy, except UK Araucania-Patagonia, but that score (and the fate of the British Empire itself) is being settled in the second world war, which shall see the USA conquer the British Dominions, among other events.