I've been running a sandbox Pathfinder game with OSR sensibilities for some time. Characters are between 1st and maybe 6th level (it is not really my business to know what level the characters are). The following are observations about how low-level characters, including spellcasters, have been useful in the game, where there typically is a mix of character of different levels in play at once. A wizard without spells is essentially a character of lower level than its companions, so I feel this is relevant.
The question does not describe the style of play of the group in any way; if you are on a railroad, or facing a set number of balanced encounters, then some of this advice is likely to not be useful, and might even be disruptive. Discuss the style of play with your game master, if necessary.
Before combat, prepare and predict
This is the most important step. You have intelligence and presumably knowledge skills, and maybe even divination spells. Ask questions like: What kinds of dangerous animals live here? What kind of terrain are we moving into? Are there any humanoids around here? What kinds of slimes and molds prefer limestone dungeons? Are such dungeons prone to having hidden pits or structural weaknesses, or are there fragile walls there? What legends are there about Magnififex and did they have particular servants when alive? What gods did they worship and what are the symbols, servant animals, and signature architecture of them?
Buy, and encourage others to buy, relevant equipment. Prepare spells accordingly. Make notes and consider anything strange you meet during the adventure.
In combat, consider all your options and be flexible
Many dedicated combat characters have a large opportunity cost to doing things in which they are not specialized. You do not have this "problem" - you roll pretty much everything at around +0 when trying to make any test in combat, and deal modest damage. This gives you half a chance of succeeding at any DC 10 test (or attack against AC 10 target), one fourth a chance at DC 15 tests, and desperate chances at DC 20 tests. Estimate the expected benefit of your action and act accordingly.
Whereas the barbarian is usually inefficient if they do not attack someone, you might try aiding that barbarian, tripping their target (with a reach weapon or after they no longer have attacks of opportunity available), bull rushing someone down a cliff, throwing caltrops on someone's way, etc. You are not that effective anyway, so there is room for using all the fun, but rarely used, tools.
Consider a crossbow.
Attack where the enemy is weak
Grab a familiar and take them hostage. Grapple the enemy wizard. Attack the goblin servants and stay clear of the ogre boss.
Reach weapons and melee
In addition to using ranged weapons, you might also consider a reach weapon. This allows you to attack into melee from some measure of safety, and also discourages enemies approaching you. If you can set the weapon against charge, it is even better. Also consider readying an attack against the first enemy to approach you. This should deter many opponents with only 1 HD, as many of them do care for their life.
Also recall that you can try to use combat maneuvers with reach without suffering an attack of opportunity (unless the enemy has reach, too). This might be useful against small or weak targets, such as other spellcasters.
Aiding another only requires a DC 10 attack roll and gives +2 to an ally. This is particularly useful against enemies with high armour class. Dress modestly, and many an enemy might ignore the feeble henchman that you are, and instead focus on the actual threat in front of them.
Do something useful besides attacking
If characters in your group have grenade-like weapons, caltrops, scrolls, potions, etc., volunteer to carry and use them. If someone is down, consider first aid to stabilize them, so that your allies can focus on the melee. Look around you (a move action) to spot enemy reinforcements, pertinent details, traps, etc. Carry a tower shield and use it to restrict enemy movement. Load crossbows for a more skilled archer. Interrogate or guard a prisoner. Intimidate. Carry the torch or the lantern.
You can, during a single round, move your speed and set a tower shield to act as a wall. If facing entrenched archers, move to protect one of you allies and set the shield. You will get total cover from the shield and your ally will get cover from you.
Use the environment
Burn, topple and snatch things. Pull the lever or push the button, if things look desperate. Consider bull rushing someone down the cliff if they have already used their attack of opportunity. Set things on fire, or extinguish them. Throw the strange powder into the campfire close to the enemies and hope it does something interesting.
Pulling it together
Prepare for combats by trying to get useful equipment and positioning yourself, and your allies, in a clever way. Climb into a tree if wolves are approaching and kill them with the crossbow.
During combat, if your team is doing well, main the status quo and use reliable maneuvers - aid another, attack low AC targets, try first aid if an ally is down so that the cleric can finish an enemy.
During a combat, if a TPK is imminent, figure out what would be enough to turn the tide and do whatever gives the largest chance for that to happen. Cause as much chaos as possible.