Some Notes About combat in OSR

Since this was my first time DMing a large combat event things got a little out of hand and my D&D 3.5 brain took over.  We had a few players move AFTER attacking and also used Attack of opportunity.  That’s a “no no” in B/X style play and while its OK to house rule stuff I want to keep combat as close to these rules as possible.

Taking into consideration the following combat sequence:

1.      Cast a spell or run from melee have to be announced before we roll initiative.

2.      Roll a d6 for initiative (Both Sides)

3.     Then whoever won: move, missile, spells, melee, in that order.

I will ask the whole group if anyone is moving and they move, then ask them if they are firing bows or whatever and everyone just makes their rolls and tells me what they got, and so on., The PCs should talk and work out a group plan for the turn. Like, coordinate moving to make room for a spell or missile fire.  Don't get in the way if you know that your team is planning to fire at the enemy. Plan your round. If you shoot a bow into melee it will be penalized depending on the scenario. (-1 to -4) 

Since there is a sequence you can't move after melee. Movement comes before melee. So you'd have to move the next round. But no "opportunity attack". B/X rules don't have them. If you win the initiative, running away from a fight is an important and viable choice.  So the player would have to decide, stay and fight then move away next round, or move away now?

There's an exception. If the monster already made its action for that round, and then you attack him, if in the next initiative roll, for the next round, you win, you can move before it attacks you. Visually, you attacked the monster and then moved away, but mechanically, your movement comes during the next round.

There are also rules for Defensive Movement like Fighting Withdrawal and Retreat.

A fighting retreat allows a character to move backwards at 1/2 normal encounter movement. However, there must be a clear path for this movement. (You could move back and drink a healing potion if you like)

A full retreat occurs when a character moves backwards at a faster rate than 1/2 of encounter movement. The character making the movement forfeits his attack this round, and his opponent attacks with a +2 to hit. (On the monsters turn, not during your own turn) In addition, if the retreating character is carrying a shield, it does not apply to the character's armor class during the retreat.

Remember: Monsters only attack on their turn. If the PC makes a full retreat and the monster decides to pursue then yes it gets a +2

Some notes on movement passed a 5’ square with a hostile in it.

Once you are within 5' of an opponent’s (melee range) you can only move away from them using defensive movement.

If the opponent is in combat with someone else, they can't follow you unless they also use defensive movement to withdraw/retreat from that combat.

So you could move into the square next to the monster on your way to wherever you're trying go, but you'd have to stop there as you would be within melee range (and "in combat" even if you aren't necessarily fighting). Next round, you could use defensive movement to withdraw from combat and keep moving. (Maybe just don't go that way if you don't want to fight)

This is an excellent link that explains most of the above in greater detail:
https://basicdungeonsdragons.blogspot.com/2017/05/an-interpretation-of-basic-d.html