In order to include op4j in your maven-managed project, you will have to declare your dependency like this:
<dependency>
<groupId>org.op4j</groupId>
<artifactId>op4j</artifactId>
<version>[version]</version>
<scope>compile</scope>
</dependency>
And if you want to use the optional op4j-ognl or op4j-jodatime packages:
<dependency>
<groupId>org.op4j</groupId>
<artifactId>op4j-ognl</artifactId>
<version>[version]</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.op4j</groupId>
<artifactId>op4j-jodatime</artifactId>
<version>[version]</version>
<scope>compile</scope>
</dependency>