Oh Oliver. Where do I begin?
Ollie is my favorite blorbo, my primary comfort character. I even ship (a version of) my self-insert character with him. Granted, my version of him is probably not very much in line with the original intent for him, but then I only have a few seconds of one video to go off of in the first place so headcanons abound.
Ah right, the video. Oliver makes his debut (and only canon appearance) in the Markiplier short film, "Google Gets An Upgrade." In it, the character Google IRL (aka "Googleplier") upgrades and splits into four distinct Googles (much to the chagrin of the himbo-coded Bing IRL); the original in blue, and the others in red, yellow, and green.
This short video lets me glean one thing about "Google Yellow" compared to the others; he's shorter. By like two inches. That's really about it.
When "Google Gets An Upgrade" was first teased, with a screenshot of "the yellow Google," the tag "Googleplier" got erroneously auto-corrected to "Oliver." The fandom, of course, immediately noticed, and Mark ultimately conceded that, "well i guess he does look a little bit like an Oliver..." And with that, a new canon detail was born!
So we know he was part of the original Google IRL's update, he's shorter than the others, and his name is Oliver. That's where canon facts end.
Of course, where canon was lacking, the fandom picked up the slack, as fandoms do.
It was somehow sort of collectively decided that Oliver is "the sweet one," the friendliest and most human of the four. The other Googles personalities vary from fan creator to fan creator, but almost everyone agreed that Oliver is a ray of sunshine.
He also unfortunately became a fairly popular 'whump' target, as people like to write angst about him, so there's that. :P