They're not on the main page because they're lowrises. Only highrises which are built, under-construction or proposed are displayed in a city's diagram by default.
This can be fixed by simply checking the "lowrises" option under "structure type" in the search form at the bottom of the diagram page and hitting "search".
Spend a little bit of time getting to know the diagram search form, and it can be a lot of fun.
Oklahoma City's diagram including lowrises
Timeline of Oklahoma City's tallest buildings, measured in feet
Keep in mind that diagram URLs expire after a period of time with no activity.