The best meter for measuring both dissolved oxygen and conductivity with automatic salinity adjustment is the Hach HQ40d.  

The meter itself is item number 1134153

Hach HQ40d Multi Meter

http://www.camlab.co.uk/hq40d-multi-meter-with-two-channels-for-ph-conductivity-and-do-p12674.aspx

 LDO probes

http://www.camlab.co.uk/intellical-ldo-sensors-for-hach-hq30-and-hq40-meters-p16481.aspx

 Conductivity probes

http://www.camlab.co.uk/hach-intellical-conductivity-electrodes-p16480.aspx

 For the LDO probe:

The cap needs to be replaced each year and the I-button which has the calibration data for the new sensor cap. This is item 1161233.

http://www.camlab.co.uk/replacement-ldo-sensor-cap-intellical-sensor-for-hqd-series-meters-p17264.aspx

The Hach LDO probe is virtually maintenance free and a new cap is required once a year to maintain performance.

Other DO meters need a membrane/solution change on a regular basis which can be fiddly to get right, and optimal conditions for maintaining the probe.