An essential part of answering the OP is to answer what power controls California and Oregon and how did the US come to lose out (if at all) on controlling that territory. It is also possible that the US is limited to its pre-1800 borders, east of the Mississippi.
Nevertheless, I'll assume we're talking a POD post 1816 (so that the War of 1812 is also not an issue), if not later. Even so, this late date guarantees that New Spain / Mexico will have some relative degree of instability. It of course does not guarantee the outcome of that instability nor its permanence. It's quite possible that New Spain manages to stay just powerful enough to keep the US from acquiring the Mexican Cession. It's a bit easier to get Oregon to stay more British than OTL, but it will be difficult to posit that the US tolerates as small a Pacific border as it might have if it had granted the British a border at the Columbia River (i.e. the coast of modern day Oregon). To completely eliminate that border, you'd probably need an outright war between the US and Britain. This isn't all that difficult to arrange, however. So far, we have a somehow stable Mexico (let's assume independence from Spain by 1830 at the latest), holding California, and a war between the US and the UK which results in the loss of all of Oregon Country to the Britain.
Before we can even consider its effect on US Pacific interests, though, this will have profound effects on the US at home. First, there's no annexation of Texas or Mexican Cession to prompt further concern about slavery after the Missouri Compromise. Second, expansionism (popular most in the South) has been dealt a serious blow. Third, the US has lost a war to Great Britain. The response to the last of these could provoke one of two primary responses: the US decides to centralize in order to avenge its defeat or the US decentralizes, as North (which has born the brunt of two wars) becomes more isolationist and the South demands the conquest of land suitable for the expansion of slavery. To accomplish the OP, you'd probably need the later, with an early, successful secession of the South. However, if say, the movement to free slaves succeeds in Virginia in the 1830s, that could well be changed.
From this point on, the question becomes, Is there another North American nation that has significant coastlines on both the Pacific and the Atlantic and that is politically stable, economically dynamic, and growing enough to take the geopolitical place of the US? There are two possibilities, either Canada or Mexico. (Canada's relative small temperate zone is compensated for by the fact that Britain's sphere of influence may well include Central America and even Mexico, giving it de facto hegemony over North America, doubly so with a successful CSA and/or a collection of Mexican successor states rather than single powerful one).
In the short term of the 19th century, though, Britain will probably de facto become more powerful in the Pacific than OTL. IMO this will increase British tensions with Russia: first, there's the status of Alaska (the Russians probably wouldn't sell to the British; the British would rather have to take Alaska by force or at a bargaining table). Second, Russia is the only other European power with significant power projection capabilities in the Pacific (unless the Dutch join a Grossdeutchland) until late in the 19th century (when you'd expect the French to have caught up). There is a possibility for a resurgent Spain, but that either comes late in the game (and is probably part of some wider alliance) or is pretty hard to manage. A wider war between Russia and Great Britain isn't difficult to manufacture, and if so, the Pacific may well feature as a larger theater.
The next most significant variable is whether (and if so, how) China and Japan might emerge as industrial nations. The lack of the US' Pacific coast here won't have big effects until a bit later, but even OTL the Japanese were considerable enough that the UK signed an alliance with them, in order to make certain their navy didn't become a threat. IMO, one of the two countries will pull itself up and it will eventually grow to challenge Great Britain in the early 20th century for dominance in the Pacific. Depending on the details, Britain might prevail, but if it gets caught up in a wider war in Europe (i.e. a World War I happens), it will be hard. Even if a Pacific War doesn't happen as part of that initial conflict, the European War itself has the potential to significantly exhaust British resources (as OTL's WWI did) limiting Britain's financial strength and thus its staying power in later contests. Here is where the lack of a US may be most telling. Assuming that Britain is eclipsed (and in this TL while Britain herself may wane, a much more potent commonwealth including the white, settler Dominions / Kingdoms could continue to have significant power projection into the late 20th century), if there is not a natural successor to the US in North America, than odds are an Asian power could rise to dominance in the Pacific. There's potential for a South American candidate, but such a country doesn't have the proximity to European markets (and consequent likely economic strength) that North American candidates might offer.
All of which is to say, that it's hard to simply say that Pearl Harbor wouldn't have happened or that there would be no Open Door Policy, because such a huge dynamic of geopolitics would have been changed (and a good chunk of US history altered).