要实现AWS自动扩展AMI并安装OSSEC，您可以使用AWS CloudFormation来定义和部署您的基础架构。以下是一个示例CloudFormation模板，其中包含了自动扩展的AMI和安装OSSEC的脚本：

AWSTemplateFormatVersion: '2010-09-09'
Description: AWS Auto Scaling with OSSEC

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: 10.0.0.0/16
      EnableDnsSupport: true
      EnableDnsHostnames: true

  Subnet:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.0.0/24

  InternetGateway:
    Type: AWS::EC2::InternetGateway

  VPCGatewayAttachment:
    Type: AWS::EC2::VPCGatewayAttachment
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway

  RouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC

  Route:
    Type: AWS::EC2::Route
    DependsOn: VPCGatewayAttachment
    Properties:
      RouteTableId: !Ref RouteTable
      DestinationCidrBlock: 0.0.0.0/0
      GatewayId: !Ref InternetGateway

  SubnetRouteTableAssociation:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref Subnet
      RouteTableId: !Ref RouteTable

  SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Security group for auto scaling instances
      VpcId: !Ref VPC

  LaunchConfiguration:
    Type: AWS::AutoScaling::LaunchConfiguration
    Properties:
      ImageId: YOUR_AMI_ID
      InstanceType: t2.micro
      SecurityGroups:
        - !Ref SecurityGroup
      UserData:
        Fn::Base64: |
          #!/bin/bash
          yum update -y
          yum install -y ossec-hids-server

  AutoScalingGroup:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      LaunchConfigurationName: !Ref LaunchConfiguration
      VPCZoneIdentifier: !Ref Subnet
      MinSize: 1
      MaxSize: 3
      DesiredCapacity: 2
      Tags:
        - Key: Name
          Value: AutoScalingInstance
          PropagateAtLaunch: true

Outputs:
  AutoScalingGroupName:
    Value: !Ref AutoScalingGroup

注意替换模板中的YOUR_AMI_ID为您自己的AMI ID。

这个CloudFormation模板将创建一个VPC、子网、Internet Gateway、路由表、安全组、启动配置和自动扩展组。在启动配置中，通过UserData属性运行一个使用yum命令来安装OSSEC的脚本。

您可以通过AWS管理控制台或AWS CLI来部署这个CloudFormation模板。