If you look at ISO 8573 there are 9 parts associated with it.
With the 9 parts, Part 1 deals with the limits and acceptance criteria for each of the compressed gas classes, ie compressed air depending on requirement (Class 2.2.1 oil free compressed air).
Part 2 then details the test methods for oil content
Part 3 for humidity
Part 4 for solid particle count
Depending on application you will need equipment that will detect oil vapour content, dewpoint/moisture level in situ,
Particle counts (should be determined using a light scattering instrument(genearlly a Climet or lazer particle counting machine or similar).
Also depending on application Drager tubes may not give results that are accurate for oil, you should record both oil content( membrane filters and FTIR spectoscopy and oil vapour (PID analyser)
Note that if you are using external contractor to complete testing ensure that method statements are provided and that results are documented as per ISO 8573 criteria. They should also document how they propose to reduce system pressure as the “pressure reduction method shall not influence theresulting partice count and particle distribution”.
Test reports should follow the statement format listed in the sections of ISO statement format.
Also important to document calibration info of testing instruments in the final test report.
Samples of test reports are provided in the ISO 8573 standards.
I have used both contract companies and developed in house test methods, depending on frequency and no points, validation/requalification you need to decide whether it will be done in house or externally.
Finally if aspectic you need to test for both viable and non- viable particles.
Hope this helps