We dive there very regurally, in fact our boat is slipped there. As Divemaster Candidates the shop we're with does almost all of it's training at Lake Ouachita, in fact we just had an OW class this weekend. Conditions were fairly nice though visibility was some what disappointing due to all the boating activity (apparantly there are a lot of people out there for a week or more at a time right now). The water temp is about 76-78 on the surface and about 72-74 down to 30' where the first thermocline normally resides, below that thermocline it drops to roughly 68-70. Vis above the first thermocline was approximately 10' to 15' depending on whether or not someone had mucked the area up or not and there was little to no current down to 30' (all this info is based on an area north of point 40).
Lake Ouachita is a great lake to dive and vacation on with several different resorts to stay at, or if boating out to an island and camping is more to your liking there are many many different islands to camp on, though you have to really time it right to get some of the better ones. There are at least 2 dive shops that I am aware of on the lake, one at Brady Mountain Marian, which is fairly close to the dam that 9mm suggested (great sugestion by the way, haven't dove it myself yet, but have had a lot of friends that have and they highly recommend it) and the other shop being at Mountain Harbor Marina and Resort, both have great reputations as dive shops, though as far as I know neither offer NITROX (Enriched Air) air fills, if your certified and looking for that, you may want to bring some tanks with you or make some arraingments at one of the shops in Hot Springs, which is about 25 miles from Brady Mountain (About 30 minutes drive time).
I hope I haven't bored you with too much information, and maybe some of it will be of use to you.