So I enjoy the ease of character creation and straight forwardness of ADnD, but prefer the unified mechanic and "bounded accuracy" of DnD Next.
Here is what I've wrought (its not like I'm reinventing the wheel):
I use the Ability Score Bonus scale from DnD Next [though you could easily switch it to 3 (-3), 4-5 (-2), 6-8 (-3) 9-12 (0), 13-15 (+1), 16-17 (+2), 18 (+3)], as well as use the Proficiency Bonus that applies to weapons, skills, tools, and saving throws that you are trained in. For simplicity sake I allow a character to simply choose three Skills (or simply use Secondary Skills) and two Saving Throws to be Proficient in.
I also have all one-handed weapons dealing 1d6 damage and all two-handed weapons dealing 1d6+2 damage which essentially allows weapons choices to be cosmetic. However, if your using a weapon not on your list, don't add your Proficiency Bonus.
Here are how the Class Features are used and how I change them:
Assassin is per OSRIC, except gain +2 to Damage, when an Assassination is attempted the target makes a Dexterity Save vs a DC of 8 + Assassin's Dex Mod + Proficiency Bonus finally they gain Disguise at their Proficiency bonus with an additional +5 at Level 1.
Cleric is per OSRIC, except gain +1 to Hit with their God's chosen weapon and Turn Undead using the following rules:
As an action, you present your holy symbol and speak a prayer, each undead creature within 25 feet of you must make a Wisdom saving throw vs DC 8 + your Wisdom modifier + your Proficiency bonus. If the creature fails its saving throw, it is turned for 1 minute or until it takes damage. If such a creature has a hit points equal to or lower than your cleric level multiplied by 5, the creature is destroyed instead of turned. A turned creature must spend its turns trying to move as far away from you as it can, and it cannot willingly move closer than within 25 feet of you.
Druid is per OSRIC.
Fighter is per OSRIC, except they gain +1 to Hit and Damage, replace Fighting the Unskilled (I hate this ability, I don't know if it appeared here or in Castles and Crusades first) with a +4 to Initiative and choose a third Saving Throw to be Proficient in.
Illusionist is per OSRIC.
Magic-User is per OSRIC.
Paladin is per OSRIC, except they gain a +1 to Hit and Damage, Improved Saving Throws gives a +1 bonus to all Saving Throws at Level 1.
Ranger is per OSRIC, except +1 to Hit and Damage, Tracking uses the Proficiency Bonus and gains an additional +5 at Level 1.
Thief is per OSRIC, except a Thief gains their Proficiency Bonus to Climb, Find Traps, Hide In Shadows, Move Quietly, Open Locks/Disarm Traps, Pick Pockets, and Read Languages; they also pick two of these skills and gain an additional +5 Bonus at Level 1. Also, ignore Dexterity and Racial Adjustments to skills.
To save vs a Spellcaster, the target must make an appropriate Ability Saving Throw vs a DC of 8 + Spellcasting Ability Mod + Proficiency Bonus (e.g. to save vs a wizard's fireball make a Dex Saving Throw vs a DC of 8 + 2 (Int of 14) + 3 (Proficiency Bonus for 7th Level) or 13.
Skill List if you choose to use it: