It depends a great deal on which AB PLC you are using
In the SLC-500 series PLCs and the MicroLogix 1000 the only one shot instruction is the OSR. It is an input instruction. You need to be aware that on SLC5/01 and SLC5/02 PLCs this instruction has some restrictions on how it can be used, particularly in rungs with branches. The restrictions are covered in detail in the SLC 500 instruction set reference manual available at AB's online documentation library.
In the PLC/5, the MicroLogix 1100, 1200, 1500 there are three one shots. The ONS instruction is an input instruction and OSR and OSF are output instructions. The output bits for OSR and OSF instructions program a little differently in the PLC/5 than in the Micrlogix PLCs but otherwise function basically the same way.
In the control logix PLC there are five one shot instructions: ONS, OSR, OSF, OSRI, and OSFI. ONS, OSR, and OSF are similar in operation to the ones on the Micrologix PLCs. OSRI and OSFI are one shot function blocks.
When the input conditions preceeding a one shot rising instruction are true then the instruction operates like this:
If (Storage_bit) then
Output_bit:= false
else {
Storage_bit := true
Output_bit:= true
}
If the rung consition preceeding the one shot rising instruction are false, then the instruction does this:
Storage_bit := false
Output_bit := false
So, when the rung becomes true after it was false, the storage_bit is not set, so the storage bit is turned on and the output bit is turned on (Or true is passed in the case of the inline ONS instruction). On the next scan, if the rung is still true, the instruction finds that the storage bit is already set so it sets the output false - thus the output bit is on for only one scan.
A one shot falling bit works in a similar fashion, except it triggers the output bit for one scan when the rung changes from true to false.
Since you have asked this question for learning purposes, here is a little homework for you: Construct a short program that creates a one shot rising bit without using the ONS, OSR, OSF instructions, using only instructions from the set XIC, XIO, OTE, OTL, OTU.
Feel free to search the site, because constructing generic one shot logic has been discussed quit a bit.