When talking RPO’s the first thing everyone mentions is the bubble. However, there are a ton of different passing concepts you can incorporate including but not limited to: sit downs, hitches, verts, tunnel screens, and stick/draw which many coaches love but won't be visited here.
Below you can see a GIF of the slant-bubble. The very classic RPO most teams use and install first in their RPOs. When talking to the NZone coaches this is their base RPO. In the classic slant/bubble the QB will count the box, check the shell (# of high safeties), and find the alignment of the flat defender. The GIF below is really a run read but the QB throws the bubble anyway.
When the nickel over the slot (whether a LB, up-safety, or CB) starts playing press, you can't stick to your typical slant/bubble. Many coaches employ this "pop" RPO. The slot just runs a sit-down which means to sit down in empty space. your outside receiver to that side should run a hitch (here he runs a slant). It is great for catching the flat defender on his heels.
As you can see in the GIF below, teams will start playing the CB on the outside WR off about 8-10 yards (especially when you've ran 4 verts or a slant/wheel) and the slot will have a NB/LB/US playing pressed. When then QB/WRs see this, they have to signal to each other a switch to the tunnel from slant/bubble. Your slot will block the near-flat defender while the outside WR runs a tunnel back to the QB.
Another variation is a straight stalk block from the outside receiver and to get the bubble out there quick. You can use this when the slant just hasn't been open anyway and you want a sure-fire block on the outside for your slot so he's not blown up.
Here, obviously when you've thrown a ton of slant/bubble or stalk/bubble RPO combos, you need to find an alternative RPO that can take advantage of teams loading the box and pressing the slot inside the +10. Here we employed a fake bubble / vert combo.