diff --git a/src/bson.zig b/src/bson.zig
index 35b57ab..2dd3828 100644
--- a/src/bson.zig
+++ b/src/bson.zig
@@ -494,12 +494,16 @@ fn copy_values(arena: std.mem.Allocator, items: []const Value) std.mem.Allocator
 
 pub const ObjectIdGen = struct {
     random_prefix: [5]u8,
-    counter: u32,
+    // Atomic so concurrent connections (e.g. two hellos, or a hello racing
+    // an insert) can share one generator without a data race. The counter
+    // only needs uniqueness within a second + random prefix, so monotonic
+    // fetchAdd is fine.
+    counter: std.atomic.Value(u32),
 
     pub fn init(io: std.Io) ObjectIdGen {
         var self: ObjectIdGen = undefined;
         io.random(&self.random_prefix);
-        self.counter = 0;
+        self.counter = .init(0);
         return self;
     }
 
@@ -509,8 +513,8 @@ pub const ObjectIdGen = struct {
         const secs: u32 = @truncate(@as(u64, @intCast(now.toSeconds())));
         std.mem.writeInt(u32, oid[0..4], secs, .big);
         @memcpy(oid[4..9], &self.random_prefix);
-        self.counter +%= 1;
-        std.mem.writeInt(u24, oid[9..12], @truncate(self.counter), .big);
+        const n = self.counter.fetchAdd(1, .monotonic) +% 1;
+        std.mem.writeInt(u24, oid[9..12], @truncate(n), .big);
         return oid;
     }
 };