Pick random value with condition R

I have one dataset OL looking like that:

          RLength RName
1:        154 3V1A
2:        154 TOR7
3:        154 F238
4:        154 RQ2D
5:        154 6TA2
   ---                 
27190:         56 R015
27191:         56 OTM2
27192:         56 36BY
27193:         56 BOMI
27194:         56 C5LZ

(The size varies from 30-170 randomly)

and I have my main data XKEY looking like:

             RLength RName
  1:          31 9NHI
  2:          31 QEYA
  3:          31 VR9K
  4:          31 TZSO
  5:          31 VUM8
     ---                 
1472803:          74 SF7I
1472804:          74 THAV
1472805:          74 UKWK
1472806:          74 OGIS
1472807:          74 SCJX

(Those are different RName with size in the same range)

Then I want to take a random subset of value in XKEY corresponding to the Length of OL. For example, if have 196 RName with a length of 154 in OL: I want to pick a random subset of rows (no duplicates) in XKey with a length of 154 (Knowing that there is 100% of time the required amount in XKey).

How can I do that in one loop without asking too much memory ? I wanted to go through a for each loop but that would probably be not efficient and there is probably a function that ?

Thanks a lot,

Cheers