Introduction:
Instead of extending a combo (post B!) for wall carry or max damage, it's possible to end a combo early to set up your opponent for 50/50 oki. On a stage with walls, it's often the case that using a spike oki 50/50 followed up by a mini-combo with wall carry will guarantee a wall splat.
d/f+3 spike oki:
d/f+3 spike based oki is fairly basic, but it's also very effective. 3,1 will catch quickstand as well as back roll and will grant small combo on hit, such as 3,1, 1,3,2, B!, 3,1,2 or 3,1, 1, 1, f+4,4,3. d+3+4 will catch some rolls and hit them if they decide to stay grounded. Unfortunately it's possible to escape both options by doing a delayed side roll and stay grounded after the roll.
b+4,2 wall spike:
Similarly to d/f+3 post B!, b+4,2 is a type of spike oki. Instead of setting up 3,1 and d+3+4 50/50, it sets up a u/f+3 and d+3+4 50/50. The d+3+4 will hit players that stay grounded and u/f+3 will catch players that quick stand to try and guard the d+3+4. Unlike d/f+3 spike oki, the b+4,2 has no real major risk. Namely it's possible for the opponent to recover between the two hits of 3,1 such that the first hit misses and they can duck and launch the second hit. u/f+3 on the other hand is completely safe on block and if you're feeling frisky, can immediately duck to WS+1+2 if you anticipate jabs.
d+3,4 spike oki:
Unlike the two mentioned above, d+3,4 does not set up guaranteed 50/50 options. Instead it's used as a mind game when the opponent is not normally expecting a spike. The two notable options for d+3,4 enders are CH d/f+1,4 and post u/f+4. A third more situational use is on P2 side during a normally BnB combo, namely d/b+3, CC, d+2, WS+4, d+3,4. In all cases the opponent is expecting to be thrown far back, since all these attacks can't used as a standard B! combo, but instead d+3,4 can keep the opponent close and possibly cause them to panic. Once again anticipation is your best tool in punishing the opponent appropriately in such a situation.