O

581. of

582. on

583. or

584. one

585. other

586. out

587. oil

588. only

589. our

590. over

591. old

592. off

593. order

594. open

595. own

596. often

597. once

598. office

599. object

600. outside

601. occasion

602. opportunity

603. opinion

604. offer

605. officer

606. obvious

607. opposite

608. official

609. observe

610. origin

611. otherwise