HO and OO scales are the most common scales in the model railway hobby as they offer the largest number of products to the hobbyist. However, the two names, and being interchanged with each other, often causes some confusion as to what the differences are and if they are compatible with one another.
It’s important to note that both HO and OO Scale are compatible with each other as they share the same gauge of the track (16.5mm, which is the distance between the rails). This means you can interchange locomotives, rolling stock, track and accessories of both scales and won’t experience any running concerns.
The difference between the two scales comes down to size. HO Scale models are built to 1:87th scale, meaning 87 packed together will in effect be the same size of the real thing. This is the correct scale for 16.5mm gauge track. OO Scale models are built to 1:76th scale (again, 76 crammed together will recreate the same height), however, this is not true to scale when using 16.5mm gauge track.
The main difference modellers will spot between the two scales is the size (but sometimes it takes a fine eye to pick it). OO Scale will appear slightly larger to HO Scale models and accessories. 

Yes, you can. Both use the same Gauge, which is 16.5mm.

Yes, you can, but you may need to fit alternative couplings to rolling stock and locomotives so that they will connect together. OO Scale manufacturers predominantly use the tension lock style of coupling, whereas HO Scale manufacturers choose to use more prototypical knuckle type couplings.

This is a personal preference. OO Scale is predominately seen across British models and layouts, whereas HO Scale is predominately seen across American, Australian and European models and layouts.

OO Scale has been well established in the British market for several decades and has become the norm. While a few manufacturers in Britain have tried to push towards a HO scale, this hasn’t been a welcomed addition and OO Scale continues to thrive.

This is a frequent question among the model railway community. This document is aimed to help you understand the differences without the need to study lots of books or articles.
To understand this question, let's first take a look at what the difference is between ANALOGUE and DIGITAL operation
DC ANALOGUE SYSTEMS
DC Analogue is a control system standard to most starter sets which allows the user to control the track (and not the locomotive). This means that two or more trains on the same track will move, speed up, slow down and stop at the same time. DCC DIGITAL SYSTEMS
DCC Digital stands for ‘Digital Command Control’. DCC Digital systems allow individual locomotives and accessories to be independently controlled by the controller. Other functions, such as lighting and sound, can also be independently controlled.

DCC READY
This logo indicates that the locomotive, train set or train pack is fitted with a socket that is ready to accept a DCC decoder. The logo is prominent on the item's packaging and is usually next to the picture of the locomotive or the item's barcode. It will usually be accompanied by a number, such as 6, 8, 18, 21, etc., which indicates the type of decoder this locomotive accepts (more on this below).
The locomotive requires a DCC decoder to be fitted to the socket in order for it to work on a DCC controlled layout. It can however still be used on the conventional DC (analogue) layout.

DCC FITTED
This logo indicates that the locomotive, train set or train pack is fitted with a socket that has a pre-installed factory decoder fitted to it.
This means that the locomotive is ready to run and operate on a DCC controlled layout and can now be called up by request from a Digital Controller. Additional features, such as lighting and sound, can also be independently controlled (only if the locomotive is fitted with these accessories).
Many DCC Fitted locomotives can also be run on DC layouts (see note below). Some accessories may continue to work automatically, such as lighting and sound, however, the user will not have independent control of these under DC operation. It is important to note that manufacturer's decoders who support sound playback on DC operation will only provide a selection of sounds when the locomotive is detected to be moving or stopping (such as chuffing, coasting and braking sounds).
*Some manufacturers may issue a warning that they do not recommend a DCC fitted locomotive to be operated on a DC (analogue) layout, and may void any warranties. Check your locomotive's manual for more information. Not all sound decoder chips enable sound playback on DC operation, and this information is provided in your decoder manual.

DC ANALOGUE (no logo)
Where a logo does not appear, this indicates that the locomotive, train set or train pack is not fitted with a socket to accept a DCC decoder. Some locomotives can be converted to DCC by wiring the decoder directly. This requires soldering the wires of the decoder directly to the locomotive's motor, power pickups and lighting/sound accessories.

PIN NUMBER
For DCC Ready locomotives, train sets or train packs, often a number icon will appear next to the DCC Ready Logo. This indicates the type of decoder that will fit into this DCC socket. In this example, a 21-pin type decoder is required. If the number is 8, this would indicate an 8-pin type decoder is required.