gistfile1.scala

object Test {

  case class A(name: String, value: String = "value")

  case class B(name: String, value: String = "value")

  case class C(name: String, value: String = "value")


  implicit val fullAFormater: OFormat[(A, Seq[(B, Seq[C])])] = ((JsPath \ "name").format[String]
    and (JsPath \ "value").format[String]
    and (JsPath \ "bs").format[Seq[(B, Seq[C])]])({
    (name: String, value: String, bs: Seq[(B, Seq[C])]) => (A(name, value), bs)
  }, {
    t: (A, Seq[(B, Seq[C])]) => (t._1.name, t._1.value, t._2)
  })

  implicit val fullBFormatter: OFormat[(B, Seq[C])] = ((JsPath \ "name").format[String]
    and (JsPath \ "value").format[String]
    and (JsPath \ "cs").format[Seq[C]])({
    (name: String, value: String, cs: Seq[C]) => (B(name, value), cs)
  }, {
    t => (t._1.name, t._1.value, t._2)
  })

  implicit val cFormatter = format[C]

  def test {
    val obj = (A("a"), Seq((B("b1"), Seq(C("c11"), C("c12"))), (B("b2"), Seq(C("c21"), C("c22")))))
    val json = Json.toJson(obj)
    println(json)
  }
}

It's due to order of implicits... fullAFormater requires an implicit Format[C] but it's declared afterwards which is not good.
The compiler can compile it for an unknown reason but then you get a weird NPE.

here is the code a bit refactored!

object Test {

  case class A(name: String, value: String = "value")
  case class B(name: String, value: String = "value")
  case class C(name: String, value: String = "value")

  implicit val cFormatter = Json.format[C]

  implicit val fullBFormatter: Format[(B, Seq[C])] = (
     (JsPath \ "name").format[String] and
     (JsPath \ "value").format[String] and
     (JsPath \ "cs").format[Seq[C]]
  )(
   { (name: String, value: String, cs: Seq[C]) => (B(name, value), cs) },
   { t => (t._1.name, t._1.value, t._2) }
  )

  implicit val fullAFormater: Format[(A, Seq[(B, Seq[C])])] = (
    (JsPath \ "name").format[String] and
    (JsPath \ "value").format[String] and
    (JsPath \ "bs").format[Seq[(B, Seq[C])]]
  )(
    { (name: String, value: String, bs: Seq[(B, Seq[C])]) => (A(name, value), bs) },
    { t: (A, Seq[(B, Seq[C])]) => (t._1.name, t._1.value, t._2) }
  )

  def test() = {
    val obj = (A("a"), Seq((B("b1"), Seq(C("c11"), C("c12"))), (B("b2"), Seq(C("c21"), C("c22")))))
    val json = Json.toJson(obj)
    println(json)
  }
}

BTW OFormat is quite technical, just use Format if you don't need OFormat explicitly.