I need to securely paired a bluetooth device to an iOS device, I know that in Bluetooth 4.0 and 4.1 there is three way to do the pairing : Just Works, Passkey Entry and Out Of
The only available bonding (pairing) method in Core Bluetooth is passkey entry.
The Apple Watch uses an OOB technique involving an image captured by the phone's camera with a fallback to passkey, but this is not available to other apps.
If your device has a display and can generate a random passkey then this technique is much the same as OOB in terms of security. If your device doesn't have a display then the security depends on how the passkey is shared (e.g. fixed passkey such as 0000 for all devices, not very secure. Random passkey for each device on a sticker is more secure)