Ninetales is UU, but he shouldn't be underestimated in OU combat. Its primary, if not only, asset in OU matches is its ability to scare off sleep abusing leads.
Ninetales is a useful lead for scaring off common sleep leads like Exeggutor, Jynx, and the occasional Venusaur. Since the aforementioned Pokémon are slower and weak to Fire-type moves (a critical hit Fire Blast is an OHKO), the opponent is immediately put at a disadvantage. If you're confident in your predicting skills, it's possible to Body Slam common switch-ins like Starmie and pray for paralysis.
Take note of Fire Blast's high burn rate—it's possible to burn Pokémon like Starmie, leaving them immune to paralysis for the rest of the match. On the other hand, burn can be useful when the entering Pokémon is a physical attacker like Golem, Rhydon, Tauros, or some variants of Snorlax. In general, on this set, paralysis is the preferred status because you can combine it with Confuse Ray to reduce the opposing Pokémon's chances of attacking frail Ninetales.
In addition to functioning as an anti-sleep lead and a parafuser, Ninetales also makes a decent trapper. It should be noted that even though Ninetales packs a decent Speed stat, Fire Spin is too dangerously inaccurate to sweep with. It's recommended to use Fire Spin to obtain free switches, or if necessity arises, in tandem with paralysis and confusion to reduce the chances of Ninetales being damaged when it misses.
Reflect can be used to raise its mediocre Defense, and allow for more abuse of paralysis and confusion. Hyper Beam is an option if you want to get a surprise critical hit on Chansey or Alakazam.
Checks and Counters
Be wary of Starmie, who can absorb your Fire Blasts and take advantage of the burn to gain immunity to paralysis. Other Water-types like Slowbro and Lapras counter Ninetales as well, even though Lapras's Ice typing causes Fire attacks to do normal damage. Golem and Rhydon also resist all of Ninetales's moves, although neither enjoys a Fire Blast burn.
In a UU environment, Vaporeon, Kabutops, and Omastar all work well, especially the latter two since both resist all of Ninetales's most common attacks and can hit it hard with STAB Surfs.