What concerns of the play are reinforced in the Osric episode?
Several concerns are reinforced. On the level of the plot, Hamlet is prodded toward the final duel with Laertes. This shows Claudius' hand still at work.
On the character level, we see Hamlet dealing with yet another person, keeping him at bay, and Horatio staying level.
On the thematic level, we see Hamlet playing at madness again, showing his verbal skill, and, most subtly, showing the gap between words and reality.