Open registry. This is a system whereby a country may allow ships to be registered there and fly the country's flag without the real owner having any definite connection with the country. The phrase is similar to "Flag of Convenience" but that phrase is more often used in a critical manner, for example by the ITF.

Open charter.A charterparty in which neither the nature of the cargo nor the ports of destination are specified is called an "open charter".

Owners’ agents. If the consignment clause in a charterparty provides for charterers’ agents to be employed at ports of loading or discharge, it is in owners’ interests to appoint their own agents who can assist the matter in any controversy which may arise. Ordinarily charterers’ agents will protect their principals’ interests first and owners’ interests second. Should difficulties arise, the master should be in a position to apply to owners’ agents who are familiar with the custom of the port. Owner’s agents may be called “protecting agents”.

Optional cargo clause. In a time charterparty, such as in the New York Produce Exchange form, the vessel is

Of 24 hours. Such an expression may be used in the description of a "working day" and taken by charterers to mean that it comprises 24 hours, which may not be consecutive.

Out of pocket expenses. The out of pocket expenses of a vessel in operation only cover the extra expenses which are incurred in comparison with a vessel lying idle in the home port. Wages, food, etc., as well as depreciation charges in full can be deducted from the running costs of the vessel in operation, in order to arrive at the out of pocket expenses.

Overstowing. A clause in a charterparty may allow either the charterer or the owner to load cargo over the original cargo, if there is available space and deadweight capacity available. However, overstowing may cause problems with the time for discharge of the original cargo as occurred in the case of The Mexico I, 1990.  Overstowing by the owner may also lead to claims if the cargo beneath is damaged.

Open cover. Here the underwriter undertakes to insure all shipments over a specified period.

On her beam ends. This term is used to describe a vessel which has listed as a result of shifting of cargo or ballast to such an extent that her deck edge is submerged and her righting power is insufficient to restore the original position.

Overside delivery clause. Sometimes consignees of parcels are granted the option of taking delivery of the goods by their own lighters.

Other mutual insurance. A mutual protection is no longer exclusive to shipowners. Other cover for liability can be mutual, for example, for professional negligence by shipowners or the liability of freight forwarders.

Owner's broker. The shipbroker whose services are used by a shipowner alone in a negotiation. This contrasts with a charterer's agent or broker, who represents only the charterer.

Economic speed or Optimum speed. Several factors are considered when determining the economic speed of a vessel.

Of 24 consecutive hours or of 24 running hours. These are actually consecutive hours during "working days" which are not expressly exceptions from laytime and not holidays.

Paramount clause. (Also termed Clause Paramount.) This clause is generally found in a bill of lading but can also be found in a charterparty.